AMPLIFICADOR OPERACIONAL

As we've seen, negative feedback is an incredibly useful principle when applied to operational amplifiers. It is what allows us to create all these practical circuits, being able to precisely set gains, rates, and other significant parameters with just a few changes of resistor values. Negative feedback makes all these circuits stable and self-correcting.

The basic principle of negative feedback is that the output tends to drive in a direction that creates a condition of equilibrium (balance). In an op-amp circuit with no feedback, there is no corrective mechanism, and the output voltage will saturate with the tiniest amount of differential voltage applied between the inputs. The result is a comparator:

With negative feedback (the output voltage "fed back" somehow to the inverting input), the circuit tends to prevent itself from driving the output to full saturation. Rather, the output voltage drives only as high or as low as needed to balance the two inputs' voltages:

Whether the output is directly fed back to the inverting (-) input or coupled through a set of components, the effect is the same: the extremely high differential voltage gain of the op-amp will be "tamed" and the circuit will respond according to the dictates of the feedback "loop" connecting output to inverting input.

Another type of feedback, namely positive feedback, also finds application in op-amp circuits. Unlike negative feedback, where the output voltage is "fed back" to the inverting (-) input, with positive feedback the output voltage is somehow routed back to the noninverting (+) input. In its simplest form, we could connect a straight piece of wire from output to noninverting input and see what happens:

The inverting input remains disconnected from the feedback loop, and is free to receive an external voltage. Let's see what happens if we ground the inverting input:

With the inverting input grounded (maintained at zero volts), the output voltage will be dictated by the magnitude and polarity of the voltage at the noninverting input. If that voltage happens to be positive, the op-amp will drive its output positive as well, feeding that positive voltage back to the noninverting input, which will result in full positive output saturation. On the other hand, if the voltage on the noninverting input happens to start out negative, the op-amp's output will drive in the negative direction, feeding back to the noninverting input and resulting in full negative saturation.

What we have here is a circuit whose output is bistable: stable in one of two states (saturated positive or saturated negative). Once it has reached one of those saturated states, it will tend to remain in that state, unchanging. What is necessary to get it to switch states is a voltage placed upon the inverting (-) input of the same polarity, but of a slightly greater magnitude. For example, if our circuit is saturated at an output voltage of +12 volts, it will take an input voltage at the inverting input of at least +12 volts to get the output to change. When it changes, it will saturate fully negative.

So, an op-amp with positive feedback tends to stay in whatever output state it's already in. It "latches" between one of two states, saturated positive or saturated negative. Technically, this is known as hysteresis.

Hysteresis can be a useful property for a comparator circuit to have. As we've seen before, comparators can be used to produce a square wave from any sort of ramping waveform (sine wave, triangle wave, sawtooth wave, etc.) input. If the incoming AC waveform is noise-free (that is, a "pure" waveform), a simple comparator will work just fine.

However, if there exist any anomalies in the waveform such as harmonics or "spikes" which cause the voltage to rise and fall significantly within the timespan of a single cycle, a comparator's output might switch states unexpectedly:

Any time there is a transition through the reference voltage level, no matter how tiny that transition may be, the output of the comparator will switch states, producing a square wave with "glitches."

If we add a little positive feedback to the comparator circuit, we will introduce hysteresis into the output. This hysteresis will cause the output to remain in its current state unless the AC input voltage undergoes a major change in magnitude.

What this feedback resistor creates is a dual-reference for the comparator circuit. The voltage applied to the noninverting (+) input as a reference which to compare with the incoming AC voltage changes depending on the value of the op-amp's output voltage. When the op-amp output is saturated positive, the reference voltage at the noninverting input will be more positive than before. Conversely, when the op-amp output is saturated negative, the reference voltage at the noninverting input will be more negative than before. The result is easier to understand on a graph:

When the op-amp output is saturated positive, the upper reference voltage is in effect, and the output won't drop to a negative saturation level unless the AC input rises above that upper reference level. Conversely, when the op-amp output is saturated negative, the lower reference voltage is in effect, and the output won't rise to a positive saturation level unless the AC input drops below that lower reference level. The result is a clean square-wave output again, despite significant amounts of distortion in the AC input signal. In order for a "glitch" to cause the comparator to switch from one state to another, it would have to be at least as big (tall) as the difference between the upper and lower reference voltage levels, and at the right point in time to cross both those levels.

Another application of positive feedback in op-amp circuits is in the construction of oscillator circuits. An oscillator is a device that produces an alternating (AC), or at least pulsing, output voltage. Technically, it is known as an astable device: having no stable output state (no equilibrium whatsoever). Oscillators are very useful devices, and they are easily made with just an op-amp and a few external components.

When the output is saturated positive, the V_ref will be positive, and the capacitor will charge up in a positive direction. When V_ramp exceeds V_ref by the tiniest margin, the output will saturate negative, and the capacitor will charge in the opposite direction (polarity). Oscillation occurs because the positive feedback is instantaneous and the negative feedback is delayed (by means of an RC time constant). The frequency of this oscillator may be adjusted by varying the size of any component.

Practical considerations: common-mode gain

As stated before, an ideal differential amplifier only amplifies the voltage difference between its two inputs. If the two inputs of a differential amplifier were to be shorted together (thus ensuring zero potential difference between them), there should be no change in output voltage for any amount of voltage applied between those two shorted inputs and ground:

Voltage that is common between either of the inputs and ground, as "V_common-mode" is in this case, is called common-mode voltage. As we vary this common voltage, the perfect differential amplifier's output voltage should hold absolutely steady (no change in output for any arbitrary change in common-mode input). This translates to a common-mode voltage gain of zero.

The operational amplifier, being a differential amplifier with high differential gain, would ideally have zero common-mode gain as well. In real life, however, this is not easily attained. Thus, common-mode voltages will invariably have some effect on the op-amp's output voltage.

The performance of a real op-amp in this regard is most commonly measured in terms of its differential voltage gain (how much it amplifies the difference between two input voltages) versus its common-mode voltage gain (how much it amplifies a common-mode voltage). The ratio of the former to the latter is called the common-mode rejection ratio, abbreviated as CMRR:

An ideal op-amp, with zero common-mode gain would have an infinite CMRR. Real op-amps have high CMRRs, the ubiquitous 741 having something around 70 dB, which works out to a little over 3,000 in terms of a ratio.

Because the common mode rejection ratio in a typical op-amp is so high, common-mode gain is usually not a great concern in circuits where the op-amp is being used with negative feedback. If the common-mode input voltage of an amplifier circuit were to suddenly change, thus producing a corresponding change in the output due to common-mode gain, that change in output would be quickly corrected as negative feedback and differential gain (being much greater than common-mode gain) worked to bring the system back to equilibrium. Sure enough, a change might be seen at the output, but it would be a lot smaller than what you might expect.

A consideration to keep in mind, though, is common-mode gain in differential op-amp circuits such as instrumentation amplifiers. Outside of the op-amp's sealed package and extremely high differential gain, we may find common-mode gain introduced by an imbalance of resistor values.

DAHIANA ALEJANDRA ROSALES HERNÁNDEZ

EES

http://www.cybermike.net/reference/liec_book/Semi/SEMI_8.html

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The "operational" amplifier

Long before the advent of digital electronic technology, computers were built to electronically perform calculations by employing voltages and currents to represent numerical quantities. This was especially useful for the simulation of physical processes. A variable voltage, for instance, might represent velocity or force in a physical system. Through the use of resistive voltage dividers and voltage amplifiers, the mathematical operations of division and multiplication could be easily performed on these signals.

The reactive properties of capacitors and inductors lend themselves well to the simulation of variables related by calculus functions. Remember how the current through a capacitor was a function of the voltage's rate of change, and how that rate of change was designated in calculus as the derivative? Well, if voltage across a capacitor were made to represent the velocity of an object, the current through the capacitor would represent the force required to accelerate or decelerate that object, the capacitor's capacitance representing the object's mass:

This analog electronic computation of the calculus derivative function is technically known as differentiation, and it is a natural function of a capacitor's current in relation to the voltage applied across it. Note that this circuit requires no "programming" to perform this relatively advanced mathematical function as a digital computer would.

Electronic circuits are very easy and inexpensive to create compared to complex physical systems, so this kind of analog electronic simulation was widely used in the research and development of mechanical systems. For realistic simulation, though, amplifier circuits of high accuracy and easy configurability were needed in these early computers.

It was found in the course of analog computer design that differential amplifiers with extremely high voltage gains met these requirements of accuracy and configurability better than single-ended amplifiers with custom-designed gains. Using simple components connected to the inputs and output of the high-gain differential amplifier, virtually any gain and any function could be obtained from the circuit, overall, without adjusting or modifying the internal circuitry of the amplifier itself. These high-gain differential amplifiers came to be known as operational amplifiers, or op-amps, because of their application in analog computers' mathematical operations.

Modern op-amps, like the popular model 741, are high-performance, inexpensive integrated circuits. Their input impedances are quite high, the inputs drawing currents in the range of half a microamp (maximum) for the 741, and far less for op-amps utilizing field-effect input transistors. Output impedance is typically quite low, about 75 Ω for the model 741, and many models have built-in output short circuit protection, meaning that their outputs can be directly shorted to ground without causing harm to the internal circuitry. With direct coupling between op-amps' internal transistor stages, they can amplify DC signals just as well as AC (up to certain maximum voltage-risetime limits). It would cost far more in money and time to design a comparable discrete-transistor amplifier circuit to match that kind of performance, unless high power capability was required. For these reasons, op-amps have all but obsoleted discrete-transistor signal amplifiers in many applications.

The following diagram shows the pin connections for most op-amps (741 included) when housed in an 8-pin DIP (Dual Inline Package) integrated circuit:

 

Practical operational amplifier voltage gains are in the range of 200,000 or more, which makes them almost useless as an analog differential amplifier by themselves. For an op-amp with a voltage gain (A_V) of 200,000 and a maximum output voltage swing of +15V/-15V, all it would take is a differential input voltage of 75 µV (microvolts) to drive it to saturation or cutoff! Before we take a look at how external components are used to bring the gain down to a reasonable level, let's investigate applications for the "bare" op-amp by itself.

One application is called the comparator. For all practical purposes, we can say that the output of an op-amp will be saturated fully positive if the (+) input is more positive than the (-) input, and saturated fully negative if the (+) input is less positive than the (-) input. In other words, an op-amp's extremely high voltage gain makes it useful as a device to compare two voltages and change output voltage states when one input exceeds the other in magnitude.

In the above circuit, we have an op-amp connected as a comparator, comparing the input voltage with a reference voltage set by the potentiometer (R₁). If V_in drops below the voltage set by R₁, the op-amp's output will saturate to +V, thereby lighting up the LED. Otherwise, if V_in is above the reference voltage, the LED will remain off. If V_in is a voltage signal produced by a measuring instrument, this comparator circuit could function as a "low" alarm, with the trip-point set by R₁. Instead of an LED, the op-amp output could drive a relay, a transistor, an SCR, or any other device capable of switching power to a load such as a solenoid valve, to take action in the event of a low alarm.

Another application for the comparator circuit shown is a square-wave converter. Suppose that the input voltage applied to the inverting (-) input was an AC sine wave rather than a stable DC voltage. In that case, the output voltage would transition between opposing states of saturation whenever the input voltage was equal to the reference voltage produced by the potentiometer. The result would be a square wave:

Adjustments to the potentiometer setting would change the reference voltage applied to the noninverting (+) input, which would change the points at which the sine wave would cross, changing the on/off times, or duty cycle of the square wave:

It should be evident that the AC input voltage would not have to be a sine wave in particular for this circuit to perform the same function. The input voltage could be a triangle wave, sawtooth wave, or any other sort of wave that ramped smoothly from positive to negative to positive again. This sort of comparator circuit is very useful for creating square waves of varying duty cycle. This technique is sometimes referred to as pulse-width modulation, or PWM (varying, or modulating a waveform according to a controlling signal, in this case the signal produced by the potentiometer).

Another comparator application is that of the bargraph driver. If we had several op-amps connected as comparators, each with its own reference voltage connected to the inverting input, but each one monitoring the same voltage signal on their noninverting inputs, we could build a bargraph-style meter such as what is commonly seen on the face of stereo tuners and graphic equalizers. As the signal voltage (representing radio signal strength or audio sound level) increased, each comparator would "turn on" in sequence and send power to its respective LED. With each comparator switching "on" at a different level of audio sound, the number of LED's illuminated would indicate how strong the signal was.

In the circuit shown above, LED₁ would be the first to light up as the input voltage increased in a positive direction. As the input voltage continued to increase, the other LED's would illuminate in succession, until all were lit.

This very same technology is used in some analog-to-digital signal converters, namely the flash converter, to translate an analog signal quantity into a series of on/off voltages representing a digital number.

DAHIANA ALEJANDRA ROSALES HERNÁNDEZ

EES

http://www.cybermike.net/reference/liec_book/Semi/SEMI_8.html

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MÉTODO PARA LA REALIZACIÓN DE UN AMPLIFICADOR DE CAPACIDADES CONMUTADAS INSENSIBLE A LA RELACIÓN ENTRE LAS CAPACIDADES Y AL OFFSET DE LOS AMPLIFICADORES

D E S C R I P C I Ó N

OBJETO DE LA INVENCIÓN

La presente invención se refiere a un método para Ia realización de un amplificador que, utilizando técnicas de capacidades conmutadas, es insensible a Ia relación entre las capacidades y al offset de los amplificadores operacionales. El método consiste en utilizar cuatro fases de reloj y dos amplificadores operacionales en un circuito de capacidades conmutadas, de forma que, tras las dos primeras fases de reloj se almacene una estimación del error producido por el desapareamiento entre capacidades. Posteriormente, en las dos fases de reloj restantes, se realimenta dicho error para realizar una amplificación con ganancia independiente de Ia relación entre capacidades y el offset de los amplificadores operacionales. El circuito propuesto para Ia implementación de Ia invención consta de dos amplificadores operacionales (o de transconductacia) y tres condensadores, de forma que uno de ellos se utiliza para el almacenamiento del error. El método propuesto permite liberar un amplificador operacional en fases de reloj no consecutivas y realizar el muestreo y retención de Ia señal de entrada sin aumento del consumo de potencia. La invención está relacionada con los circuitos de capacidades conmutadas, muy utilizados en Ia realización de filtros en tiempo discreto y convertidores analógicos digitales. El método encuentra aplicación en diseño de convertidores analógicos digitales basados en Ia arquitectura pipelined.

ANTECEDENTES DE LA INVENCIÓN

Una de las técnicas más habituales para realizar circuitos analógicos de procesado de señal en tecnología CMOS es mediante el uso de capacidades conmutadas. Estos circuitos se componen de condensadores, interruptores y

amplificadores operacionales o de transconductancia. Entre los posibles bloques constructivos realizables con Ia técnica de capacidades conmutadas, uno de los más populares es un amplificador de ganancia controlada de forma precisa por Ia relación entre dos capacidades. La potencia consumida por estos circuitos es directamente proporcional al tamaño de las capacidades. Sin embargo, en determinadas aplicaciones (como el diseño de convertidores analógico-digitales) donde Ia relación entre las capacidades debe ser muy precisa, el tamaño de dichas capacidades debe ser suficientemente grande como para asegurar que Ia relación entre ellas toma un valor Io más cercano posible al valor esperado.=Esta razón se ha convertido en el principal obstáculo para realizar circuitos de capacidades conmutadas de muy bajo consumo y alta precisión.

Por otro lado, el offset de los amplificadores operacionales limita Ia resolución de circuito de capacidades conmutadas, obligando a Ia utilización de costosas técnicas de cancelación del offset. En este sentido cabe citar las patentes estadounidenses 4393351 y 5880630.

En los últimos años han aparecido numerosos amplificadores de capacidades conmutadas que abordan estos problemas desde distintos enfoques. En primer lugar, se pueden destacar las técnicas de auto calibración digital, en Ia cuales se compensa digitalmente el desapareamiento entre capacidades (error en el valor esperado para Ia relación entre dos capacidades) (Shang-Yuan (Sean) Chuang, Terry L. Sculley; "A Digitally Self-Calibrating 14-bit 10MHz CMOS Pipelined A/D Converter" IEEE Journal of Solid-State Circuits. Vol37, N 6, Junio 2002). La lógica de control y las memorias necesarias para Ia aplicación de estas técnicas implican un aumento importante en el consumo y área del circuito. En segundo lugar cabe destacar las técnicas de promediado del error (Bang-Sup Song; Tompsett, M.F.; Lakshmikumar, K.R.; "A 12-bit 1-Msample/s capacitor error-averaging pipelined A/D converter" IEEE Journal of Solid-State Circuits, VoI: 23 , Iss: 6 , Diciembre 1988, Páginas: 1324 - 1333). Este tipo de técnicas sólo alivia el problema, reduciendo Ia magnitud del error sin eliminarlo. Por último, es posible realizar el amplificador de capacidades conmutadas de forma que su ganancia sea independiente a Ia relación entre las capacidades. En esta última aproximación al problema podemos englobar Ia presente invención. Estas técnicas permiten reducir el tamaño de las capacidades utilizadas y consecuentemente Ia potencia consumida.

DESCRIPCIÓN DE LA INVENCIÓN

El método que Ia invención propone consiste Ia utilización de cuatro fases de reloj y dos amplificadores operacionales (o de transconductancia) para implementar un amplificador de ganancia dos insensible a Ia relación entre capacidades. Uno de los amplificadores operacionales realiza Ia función amplificadora, mientras que el otro, además de realizar el muestreo y retención, implementa Ia técnica propuesta. La operación del circuito en las cuatro fases de reloj está dividida de Ia siguiente forma: En Ia primera fase se realiza el muestreo de Ia señal de entrada y Ia inicialización del circuito. En Ia segunda almacena (para su posterior cancelación) el error debido al desapareamiento entre capacidades y el offset de los amplificadores. En Ia tercera se vuelve a muestrear Ia señal de entrada. Y por último, en Ia cuarta fase se realiza Ia amplificación y se utilizan los errores almacenados para realizar Ia cancelación. La invención propuesta tiene Ia ventaja de no necesitar el primer amplificador operacional durante las fases impares de reloj, haciendo posible Ia utilización del amplificador para otros propósitos con el consiguiente ahorro de energía.

DAHIANA ALEJANDRA ROSALES HERNANDEZ

EES

http://www.sumobrain.com/patents/wipo/Method-producing-switched-capacitor-amplifier/WO2006042888.html

http://dahianarosales.blogspot.com/2010/02/metodo-para-la-realizacion-de-un.html

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EL AMPLIFICADOR OPERACIONAL

 El op-amp tiene muchísimas otras aplicaciones.  A manera de ejemplo, a continuación tenemos un diseño en el cual el op-amp está siendo usado para construír un amplificador para una señal de audio:

Y en el siguiente circuito, tenemos un diseño que nos permite convertir una medición de temperatura en una medición de voltaje, para lo cual utilizamos un componente transductor, el conocido sensor de temperatura AD590 fabricado por la empresa Analogue Devices:



A continuación tenemos lo que en audio se conoce como un filtro activo de paso bajo:


Para un técnico en electrónica, explicar el funcionamiento de un circuito de este tipo, aún sin las fórmulas, no representa gran problema. Para frecuencias lo suficientemente bajas, el condensador (capacitor) C2 puesto en paralelo con la resistencia R2 actúa como un circuito abierto, pudiendo ser ignorado, con lo cual lo que tenemos aquí es esencialmente un amplificador inversor. Y para frecuencias lo suficientemente altas, el condensador C2 actúa como un corto circuito, con lo cual el efecto resistivo de R2 queda nulificado y la ganancia, de acuerdo con la fórmula para el amplificador inversor, se vuelve cero. En pocas palabras, el circuito amplifica y deja pasar las señales de bajas frecuencias (que en una señal de audio correspondería a los sonidos bajos o bass), y bloquea las señales de altas frecuencias. Este comportamiento es representado con una figura conocida como la curva de responsiva a la frecuencia (frequency response curve) que para un filtro de paso bajo toma la siguiente apariencia:


Muchos usuarios de equipo de audio sin saberlo ya están familiarizados con este tipo de curvas de responsiva de frequencia debido a que su equipo cuenta al frente en su panel de controles con varios deslizadores (sliders) que forman parte del ecualizador (equalizer) de su aparato de audio, e inclusive muchos programas de audio para computadora cuentan con la opción de activar y mostrar en la pantalla un ecualizador gráfico (graphic equalizer) como el siguiente (el programa de Microsoft conocido como Windows Media Player para la ejecución de archivos de audio y video en diversos formatos cuenta con su propio ecualizador gráfico activable desde la línea del menú):


La disposición de los controles deslizadores en este tipo de objetos no es casualidad alguna, los controles están situados el uno junto al otro de modo tal que tomados en conjunto sugieran el tipo de responsiva de frecuencia que se le dará a la señal de audio, como lo muestra la gráfica de responsiva de frecuencia del ecualizador gráfico arriba mostrado situada debajo de los controles; obsérvese cómo la disposición de los controles deslizadores en la parte superior parece sugerir la forma de la responsiva de frecuencia en la parte inferior del ecualizador gráfico.Es importante agregar que además del filtro de paso bajo arriba mostrado, con el op-amp también se pueden construír filtros de paso alto (los cuales bloquean las señales de bajas frecuencias permitiendo el paso de las señales agudas de alta frecuencia), filtros de paso de banda (bandpass) que permiten pasar las frecuencias intermedias dentro de cierto rango, filtros Butterworth, filtros Bessel, etc., lo cual por sí solo requiere ser tratado en un curso especializado a nivel universitario.Además del op-amp 741, hay otros op-amps disponibles en el mercado, como el LM324, el cual es un circuito integrado que contiene el equivalente de cuatro op-amps 741, cuyo bajo costo lo hace casi tan accesible como el mismo 741, y el cual tiene otra característica muy deseable: a diferencia del op-amp 741 que requiere de dos fuentes de poder, una positiva (+V) y una negativa (-V) el LM 324 sólo requiere de una fuente de poder. A continuación tenemos el diseño de un filtro activo de paso-de-banda usando uno de los op-amps del LM324, en donde es obvio que estamos utilizando una sola fuente de poder de +15 volts sin necesidad de tener que utilizar una fuente de poder de -15 volts:


El amplificador operacional, con el apoyo de unos cuantos componentes pasivos externos (resistencias y/o condensadores) puede llevar a cabo operaciones aritméticas de suma, resta, multiplicación, e inclusive las operaciones de diferenciación e integración propias del cálculo infinitesimal. Es por esto mismo que no hace mucho tiempo, todavía hasta las décadas de los setentas y los ochentas, los amplificadores operacionales junto con componentes pasivos adicionales eran utilizados como los bloques fundamentales de computadoras analógicas con el fin de poder resolver problemas matemáticos que inclusive involucraban la simulación de sistemas físicos modelados por algo conocido en el campo de las matemáticas como ecuaciones diferenciales, las cuales son fórmulas matemáticas que involucran derivadas como las siguientes:




La desventaja de utilizar amplificadores operacionales para resolver este tipo de problemas es que los resultados tienen una precisión limitada a dos o tres dígitos, porque los valores tanto de las entradas como de las salidas de las operaciones aritméticas o de los sistema físicos que están siendo simulados son esencialmente voltajes, valores medidos. Esta es la razón por la cual el advenimiento de las computadoras digitales volvió obsoleta la tecnología de los amplificadores operacionales aplicada con operaciones matemáticas en mente. De cualquier modo, para aplicaciones especializadas, es bueno saber que esta posibilidad está siempre disponible. Podemos obtener mayores detalles acerca del uso de los amplificadores operacionales en la solución de problemas matemáticos en algún enlace como el siguiente:http://www.ibiblio.org/obp/electricCircuits/Semi/SEMI_9.html#xtocid1561110Para poder obtener alguna "práctica" en el uso y manejo de los amplificadores operacionales, se le recomienda al lector visitar el siguiente sitio mantenido por el Profesor Constantinos E. Efstathiou, Director del Laboratorio de Química Analítica de la Universidad Nacional y Kapodristiana de Atenas, en el cual el usuario se puede divertir "jugando" un buen rato con amplificadores operacionales simulados:http://www.chem.uoa.gr/applets/AppletOpAmps/Appl_OpAmps2.htmlEn esta página del Profesor Efstathiou, el simulador de amplificadores operacionales aparece del lado derecho de la pantalla, y en cuyo borde inferior se puede escoger una de varias opciones para poder experimentar: Inverting Amp. (amplificador inversor), Summing Amp (amplificador sumador), Difference Amp (amplificador de diferencias) e Integrator (integrador). La forma en la cual trabaja este simulador es la siguiente: supóngase que se quiere experimentar un rato con la opción más elemental de todas, la opción Inverting Amp. Al seleccionar esta opción, a la izquierda del circuito estará activado únicamente uno de los controles "deslizadores", el control marcado como V1. A un lado del control se le dan al usuario dos opciones de rangos de voltaje, ±2 volts y ±20 volts. Supóngase que las resistencias Ri y Rf tienen valores de 10 KΩ (10 mil ohms) y 100 KΩ (100 mil ohms) respectivamente, lo cual de acuerdo con la fórmula de la ganancia para un circuito inversor dará una ganancia de -10. Suponiendo que se ha seleccionado la opción de ±2 volts, entonces si el control deslizador está situado justo en el extremo medio poniendo a la entrada del circuito un voltaje de 0 volts, a la salida en el indicador numérico simulado leermos un también un voltaje de 0 volts, lo cual era de esperarse porque no se está llevando a cabo la amplificación de nada. Pero si a la entrada ponemos un voltaje de +0.25 volts, a la salida tendremos un voltaje de -2.5 volts. Esto nos indica que el voltaje de entrada está siendo amplificado diez veces, y el cambio de signo en la polaridad de voltaje nos indica que efectivamente se está llevando a cabo una inversión en la polaridad, lo cual era de esperarse para un amplificador inversor. La ventaja del simulador es que le podemos cambiar los valores a las resistencias para cambiar con ello la ganancia del circuito. A manera de ejemplo, si a las resistencias Ri y Rf les damos valores de 20 KΩ (20 mil ohms) y 100 KΩ (100 mil ohms) respectivamente, con lo cual obtendremos una ganancia de -4, entonces si le aplicamos al circuito un voltaje de entrada de +0.25 volts obtendremos un voltaje a la salida de -0.625 volts, como era de esperarse.

DAHIANA ALEJANDRA ROSALES HERNÁNDEZ

EES

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AMPLIFICADOR OPERACIONAL

La enorme ventaja de los circuitos lógicos es que no se requiere tener muchos conocimientos de electrónica para poder comprender cómo trabajan funcionalmente dichos circuitos. Por lo general identificamos un valor de un "1" lógico con un voltaje generalmente positivo (como +5 volts) y un "0" lógico con un nivel de cero volts, el equivalente a "tierra" eléctrica que en una batería no vendría siendo más que el polo negativo de la batería marcado con el signo menos (-). Existen muchos sistemas digitales en los que la mayoría del sistema basa su funcionamiento en electrónica puramente digital. Las computadoras de escritorio son un buen ejemplo de ello. Sin embargo, existen también sistemas en los cuales es necesario interconectar componentes propios de la electrónica digital (bloques AND, OR y NOT) que siempre trabajan con valores discretos de voltaje, con componentes propios de la electrónica analógica, en donde se trabaja con voltajes que pueden variar continuamente entre dos límites pudiendo tomar cualquier valor posible entre dichos límites. Estos sistemas son esencialmente sistemas híbridos que combinan en un solo diseño aspectos de la electrónica digital y de la electrónica analógica. Este auditorio en mente que se ha preparado este Suplemento que trata sobre uno de los componentes más versátiles para el diseños de circuitos analógicos: el amplificador operacional.Así como en la familia lógica de circuitos integrados TTL el circuito integrado 7400 es el componente fundamental de referencia del cual parten todos los demás, y así como en en el mundo de los microprocesadores el microprocesador 8008 marcó la pauta a seguir por todos los demás microprocesadores que le sucedieron, del mismo modo en el mundo de los amplificadores operacionales el punto de referencia es un amplificador designado con el número 741.Si fuéramos a comprar en el mercado un amplificador operacional 741 fabricado por la empresa Motorola, dicho componente tendría el siguiente aspecto físico:

El diagrama esquemático de este circuito integrado en relación con sus ocho terminales muestra las siguientes designaciones funcionales de cada una de dichas terminales o "pins":



Esencialmente, el amplificador operacional es representado en los diagramas esquemáticos tal y como se muestra arriba, como un triángulo, con dos terminales de entrada, una entrada inversora (inverting input) identificada con un símbolo menos (-) y una terminal no-inversora (non-inverting input) identificada con un símbolo más (+).
Precaución: Los términos terminal inversora y terminal no-inversora en un amplificador operacional no tienen absolutamente nada que ver con las definiciones usadas en el mundo de los circuitos lógicos en relación con el bloque NOT.El símbolo triangular utilizado para representar un amplificador operacional encierra cómodamente para nosotros algo que es esencialmente un circuito analógico algo complejo, cuyo esquemático detallado es el siguiente:


Aunque el amplificador operacional lo podemos usar como una "caja negra" sin tener que preocuparnos por los detalles internos que muestra este último diagrama, de cualquier modo tenemos que saber cómo llevar a cabo conexiones externas al mismo para poder obtener del mismo algunas funciones analógicas que nos puedan ser de utilidad. Como su nombre lo indica, este componente es un amplificador, un amplificador de una señal analógica que puede variar continuamente entre un rango de valores. Y al amplificar la señal, lo puede hacer invirtiendo la polaridad de la señal con respecto a la señal de entrada (convirtiendo los valores de voltaje positivos a negativos, y los valores de voltaje negativos a positivos) , en cuyo caso lo usamos como amplificador inversor, o dejando que la polaridad de la señal de salida se mantenga con la misma polaridad que la señal de entrada, o sea como un amplificador no-inversor. Para poder utilizarlo en cualquiera de estas dos maneras, es necesario conectarle a cada configuración unas resistencias eléctricas externas como lo muestran los siguientes diagramas:
En el diagrama superior tenemos un amplificador inversor, y en el diagrama inferior tenemos un amplificador no-inversor. Antes de entrar en detalles sobre el funcionamiento de estos circuitos, observemos primero que para poder trabajar adecuadamente el amplificador operacional requiere de dos voltajes, un voltaje positivo +V aplicado en la terminal 7, y un voltaje negativo -V aplicado en la terminal 4. Si fueramos a proporcionar estos voltajes con baterías externas de modo tal que el voltaje positivo sea +V=+15 volts y el voltaje negativo sea -V=-15 volts, lo haríamos utilizando algo como lo siguiente:



Obsérvese en los diagramos de los dos circuitos amplificadores mostrados arriba que no es necesario conectar ninguna de las terminales del amplificador operacional al punto intermedio entre las dos baterías designado en el esquemático como la tierra eléctrica. Una fuente dual de voltajes fácil de implementar con dos baterías desechables proporcionando un voltaje positivo +V=+9 volts y un voltaje negativo -V=-9 volts sería la siguiente:


Sin embargo, como la desventaja de una fuente dual de voltajes construída con baterías desechables es que las baterías tienen una vida de uso limitada, para un diseño fijo que no se estará moviendo mucho de un lugar a otro se puede construír una fuente dual de voltajes alimentada con corriente alterna de línea como la que se muestra a continuación:


Regresemos ahora a los circuitos amplificadores. La señal de entrada una vez amplificada será proporcionada por el amplificador operacional u op-amp en su terminal de salida 6 (output). La ganancia (gain) del op-amp es la medida de la amplificación de voltaje del circuito y se define simplemente como el valor instantáneo del voltaje de salida Vout en la terminal 6 entre el valor del voltaje Vin de entrada:



De este modo, si el voltaje de entrada es de 1 volt y el voltaje de salida es de 10 volts, el op-amp estará amplificando la señal por un factor de 10: la señal de salida será diez veces más grande que la señal de entrada.Si el diseño que utilizaremos será el de un amplificador inversor, entonces usaremos el circuito designado arriba como "inverting amplifier", y la señal de entrada a ser amplificada deberá ser aplicada en la terminal 2 (inverting input). En este caso, nosotros podemos escoger el factor de amplificación mediante una cuidadosa selección de las resistencias R1 y R2. La ganancia (Gain) del circuito, como podemos ver en la fórmula anexa al circuito, será igual al valor de R2 dividido entre el valor de R1. Si queremos un factor de amplificación de cinco tantos, entonces la resistencia R2 deberá ser cinco veces más grande que la resistencia R1. Una vez escogidos los valores de las resistencias R1 y R2 el valor de la resistencia R3 estará prácticamente prefijado por la fórmula que nos dice cuál debe ser el valor de dicha resistencia (en algunos diseños, se prescinde de esta resistencia por completo). En la fórmula de la ganancia:
Gain = -R2/R1el signo menos indica que la polaridad de la señal estará invertida con respecto a la polaridad de la señal de entrada, que es justo lo que debe hacer un amplificador inversor.Si por el contrario queremos diseñar un amplificador no-inversor, entonces usamos el circuito designado arriba como "non-inverting amplifier", y la señal de entrada a ser amplificada deberá ser aplicada en la terminal 3 (non-inverting input) directamente. Aquí también nosotros podemos escoger el factor de amplificación mediante una cuidadosa selección de las resistencias R1 y R2. La ganancia (Gain) del circuito, como podemos ver en la fórmula anexa al circuito, será igual a 1 sumado al valor de R2 dividido entre el valor de R1. Si queremos un factor de amplificación de tres tantos, entonces la resistencia R2 deberá ser dos veces más grande que la resistencia R1, lo cual sumado a la unidad nos dá una ganancia de tres:
Gain = 1 + (R2/R1) = 1 + (2/1) = 1 + 2 = 3Los valores de las resistencias generalmente deben estar seleccionados en el rango de los miles de ohms (K ohms). Valores demasiado bajos de resistencias, situados por debajo de 1 Kohm, producen corrientes eléctricas grandes que pueden producirle daño al circuito, mientras que valores demasiado grandes de resistencias, situados por encima de 1 Megohm, inducen un efecto indeseable conocido como el ruido térmico (en inglés, thermal noise) o ruido Johnson-Nyquist. A continuación tenemos un amplificador no-inversor construído en torno a un op-amp en el cual se han seleccionado resistencias R1 y R2 con valores respectivos de 1K (mil ohms) y 15K (15 mil ohms), con lo cual obtenemos un factor de multiplicación de 16 sobre la señal de entrada:

DAHIANA ALEJANDRA ROSALES HERNÁNDEZ

EES

http://dahianarosales.blogspot.com/2010/02/amplificador-operacional-la-enorme.html

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Active cascode amplifier with an amplitude limiter

 

 

TECHNICAL FIELD

The present invention relates to the electrical arts and more specifically to active cascode amplifier circuits.

BACKGROUND ART

FIG. 1 is a prior art active cascode amplifier as shown and described in U.S. Pat. No. 5,039,054 which is incorporated herein by reference in its entirety. Active cascode amplifiers have both an output stage and an input stage. In FIG. 1 the input stage is represented by a Metal-Oxide-Semiconductor-Field-Effect-Transistor ("MOSFET") transistor Q 1 and the output stage is represented by another MOSFET transistor Q 2 . Attached to the gate of the output stage, MOSFET Q 2 , is the output of an auxiliary amplifier. The auxiliary amplifier, which in FIG. 1 is an operational amplifier has a bias voltage present at the positive input and the negative input is electrically coupled to node X providing a feedback loop. The operational amplifier increases the output resistance of the active cascode amplifier. Assuming a constant current source, the increased output resistance increases the voltage gain.

The active cascode amplifier operates in the following manner. When the input voltage signal, Vin drops below the threshold voltage of the MOSFET, Q 1 shuts off. In response, the voltage at node X begins to increase, which in turn decreases the differential voltage between the positive and negative terminals of the operational amplifier. As a result, the output voltage at the control terminal (gate) of the output stage transistor Q 2 decreases eventually shutting off the output stage and causing the active cascode amplifier to slew. Given enough time, the operational amplifier will saturate toward a voltage which is close to ground. When the output voltage of the operational amplifier drops to such a voltage that the gate to source voltage is less than that of the MOSFET's threshold voltage for turning on, the output stage transistor Q 2 shuts off. The output voltage of the operational amplifier continues to fall until the voltage approaches ground. Thus, the gate to source voltage falls well below the threshold voltage. When Vin then increases and goes above the threshold voltage for the transistor, such that Q 1 turns on, the auxiliary amplifier requires a period of time, referred to as a "recovery time period" for the voltage at the gate of Q 2 to increase such that Q 2 turns on. The recovery time period is shown in the graph of FIG. 2 . This recovery time poses problems for devices which require quick circuit operation. The length of the recovery time is proportional to the difference between the gate voltage when Q 2 is operational and the voltage at the gate when Q 2 is off.

One solution to this problem known in the prior art is the inclusion of a trickle current source positioned at node X as shown in FIG. 3 . The trickle current source provides a trickle current and thus a current flow path even when the input stage MOSFET Q 1 is off. This trickle current causes the output stage transistor Q 2 to remain in a partially on state. Since Q 2 is in a partially on state, the recovery time is decreased as shown in the graph of FIG. 4 as compared to the graph of FIG. 2 . One drawback of this configuration is that the trickle current source is constantly active drawing power and the power drain provides no increase to the speed of the amplifier.

SUMMARY OF THE INVENTION

In a first embodiment of the invention there is provided an active cascode amplifier circuit which includes an active cascode amplifier and an amplitude limiter. The active cascode amplifier includes an input stage, an output stage and an auxiliary amplifier and receives in a voltage input signal and outputs a voltage output signal wherein the active cascode amplifier amplifies the input voltage signal. The auxiliary amplifier is provided within the circuit to increase the gain of the cascode amplifier and has an associated output.

When the input stage shuts off, due to a decrease in the input voltage signal, the auxiliary amplifier's output voltage falls and the amplitude limiter becomes active and holds the voltage at the output of the auxiliary amplifier to a preset voltage in order to decrease the recovery time for turning the output stage on when the input voltage increases and turns the input stage on.

The voltage at the output of the auxiliary amplifier provides a voltage to a control terminal of the output stage. When the output voltage of the auxiliary amplifier falls below a threshold voltage for the output stage, the output stage shuts off. The voltage at the control terminal would continue to fall, but for the amplitude limiter circuit. Thus, by preventing the voltage at the output of the auxiliary amplifier from falling below a preset limit, the recovery time to pull up the voltage at the control terminal and to turn the output stage on is decreased when the input stage transitions from an off state to an on state.

The amplitude limiter is a circuit which may be formed with circuitry including but not limited to a diode, a MOSFET, a JFET, or a bipolar transistor.

In one embodiment, the active cascode amplifier operates in a singe ended mode. In another embodiment, the active cascode amplifier operates in a differential mode. In further embodiments, the active cascode amplifier may be a folded active cascode amplifier which operates in a single ended or a differential mode.

The active cascode amplifier may be formed from any of a number of electrical components including but not limited to MOSFETs, JFETs, bipolar transistors, diodes and any combination thereof.

When the active cascode amplifier circuit is implemented in which the input and the output are provided as differential signals, a plurality of amplitude limiters are provided in the active cascode amplifier circuit. Each of the amplitude limiters operate in much the same way as for the single ended mode, in which the amplitude limiter holds the voltage level at the output of the auxiliary amplifier and does not allow the voltage to fall.

In a further embodiment the amplitude limiter becomes active when the output stage begins to shut off.

 

DAHIANA ALEJANDRA ROSALES HERNANDEZ

EES

http://www.freepatentsonline.com/6590456.html

http://dahianarosales.blogspot.com/2010/02/active-cascode-amplifier-with-amplitude.html

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Basic Op Amps

The operational amplifier (Op Amp) is a staple item in electronic circuits and

is a building block that often is one of the main components in linear audio and
video circuitry. The op amp is basically a high gain amplifier that is used in
conjunction with feedback networks to make up a circuit whose properties are
determined by linear passive components, such as resistors, capacitors, inductors, as
well as nonlinear components (diodes, varistors, thermistors, etc). The term
“operational amplifier” comes from the use of these devices in analog computers
that were used decades ago to perform mathematical operations (addition,
multiplication, differentiation, integration, summation, etc) on input quantities. The
term has stuck and is still used, even though analog computers have largely
departed the scene, having been replaced by digital computers long ago. The
operational amplifier of today is a sophisticated device, being composed of many
transistors, diodes, and resistors, all in a chip, and packaged in various
configurations. There are thousands of types of op amps available, from flea
powered microwatt units to units capable of handling a few hundred watts of power,
from a few cents to many dollars in cost. As you may imagine, the specs and
performance requirements, as well as reliability, temperature range, and packaging,
all affect cost. Op amps that can do many ordinary jobs very well are available for
under 50 cents, owing to low cost plastic packages and large scale integration, and
high volume production. Technologies commonly used are bipolar, FET, CMOS and
combinations. Some large or high power op amps are made using monolithic
fabrication methods.

From a circuit viewpoint, for the purposes of explanation, an ideal amplifier
is used to represent an op amp. An ideal amplifier has the following properties:
Infinite forward gain, bandwidth and input impedance, with zero output
impedance, noise voltage, DC offset, bias currents, and reverse gain.  In
practice, all op amps have some bias current that flows in the inputs, this being
almost negligible for JFET and CMOS types, but more significant in bipolar types.
This current must be considered in high impedance circuits, and in DC and
instrumentation amplifiers, and in circuits that must operate over a wide
temperature range. In addition, even if you were to short the op amp inputs together
you may not get zero output voltage, but some random DC level. This DC voltage
can be considered as an equivalent DC input offset voltage present at the input. DC
offset can also be produced from equal input bias currents flowing through unequal
resistances in the inverting and noninverting input circuits. This will produce a DC
input voltage differential at the input. Some op amps have external pins to which a
potentiometer can be connected to balance out or otherwise cancel this voltage,
bringing the DC output to zero under zero signal input conditions. These are widely
used in instrumentation amplifiers and related applications where nulling or zero
adjustments are required. All amplifiers generate some noise, which is due to
thermal and semiconductor junction effects, and can be considered as an equivalent
input noise voltage. Amplifiers are available with low noise characteristics for those
applications where noise must be kept to a minimum.

 A real world op amp has a lot
of gain (>1000X voltage gain) and a fairly high input impedance (>100K). Generally
there are two inputs shown, an inverting and a non inverting input, and one output
referenced to ground (but not always, differential outputs are sometimes used in
certain applications). One of the inputs may be grounded in many common
applications where a single ended signal source is present. This is a common
situation. There are limitations on the DC levels allowable on the inputs, and
limitations on the available output voltage swing. Op amps are available that allow a
full output voltage swing between the positive (Vcc) supply and negative (Vdd)
supply. These are sometimes referrred to as “rail to rail” capable. In addition, if the
exact same voltage is present on the inverting and non-inverting inputs, ideally the
output voltage should be zero. This is not always so, and the degree of imperfection
is called the common mode rejection ratio. This is usually 60 dB or better, with 70-
80 dB as a minimum. Note that this may vary with input voltage levels to some
degree. Also, variations of power supply voltage may show up as equivalent input
signals. The degree to which the op amp rejects this is called the supply voltage
rejection ratio. It is usually better than 60 dB and typically 70 to 80 dB or better.
After all, nothing is perfect in life.
Op amp power supply connections are sometimes shown in diagrams, especially if
decoupling capacitors and resistors are necessary, but more often shown elsewhere
in the schematic, as they play no part in the primary circuit function other than to
power the amplifier. Many general-purpose op amp chips have two or four separate
operational amplifiers in one package, with common power supply connections. In
practice the ideal amplifier criteria requirements are met only approximately, but
as will be shown, close enough for most purposes. Practically, an op amp will have a
gain of 10,000 or more, an input impedance of megohms, and a 3 dB bandwidth of
several tens of hertz or more. If an amplifier has a 3 dB bandwidth of 40 Hz and a
gain of 100,000 times, this is a gain bandwidth product of 4 million hertz, or 4 MHz.
(40 x 100,000). It is advantageous in many feedback applications to have the gain
falling at 6 dB per octave or 20 dB per decade at frequencies beyond the corner
frequency (that frequency at which the amplifier gain has fallen 3 dB or 70.7
percent of its DC value). Since the op amp is used in mainly in feedback circuits
having much lower closed loop gain, these performance figures are good enough in
many cases. In fact, even a single high gain (100X) common emitter transistor
amplifier stage can be treated as an op amp if feedback is employed, with
surprisingly little error. In many cases a single transistor will work almost as well as
a more expensive op amp device. One example is a simple audio amplifier stage
from which a moderate gain (5-20X) is required. This will be shown in an example
later.

One of the most popular op amps of all time is the venerable LM741, its dual
version LM747 and their many descendents. The JFET input TLO8X series is also
very popular, coming in single (TLO81), double (TLO82) and quadruple (TLO84)
units. The TLO81 and TLO82 come in 8 pin DIP packages, while the TLO84 comes
in a 14-pin DIP package. These op amps operate well from 5 to 12 volt experimenter
supplies, and require both a plus and a minus supply. These are also cheap and
widely available. Other general purpose types are the LM324 and LM1458 (bipolar)
and LM3900, and all their variations and flavors. There are many others, but these
types mentioned are easily obtained by the hobbyist wishing to experiment with
them, and are cheap and in plentiful supply. Many manufacturers make them, so
obsolescence should not be a problem for a long time. We will use the TLO8X series
for circuit examples, as they are general purpose JFET types, allowing the use of
higher resistance values and therefore smaller capacitor values, which is often more
convienient from a design standpoint. The TLO8X series have an open loop (no
feedback used, the full gain the amp can deliver) voltage gain of over 10000 and
having JFET inputs, an input impedance of a million megohms. The gain bandwidth
product (obtained by measuring frequency where gain falls to unity) is rated at at 4
MHz for the TLO8X series. Op amps are available with gain bandwidth products to
several hundred MHz and even higher, and these are used in video and RF
applications.

http://www.northcountryradio.com/PDFs/column003.pdf

http://dahianarosales.blogspot.com/2010/02/basic-op-amps-operational-amplifier-op.html

DAHIANA ALEJANDRA ROSALES HERNÁNDEZ

EES

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Operational Amplifiers

Operational Amplifiers

OVERVIEW
•Desired features:
-Differential input
-Infinite voltage gain
-Infinite input resistance
-Zero output resistance
•Applications
-Switched capacitor
•MOS topologies
-Two stage MOS
-Two stage MOS with cascode
-MOS telescopic-cascode
-MOS folded-cascode
-MOS Active-cascode


Applications

•Feedback concept
-All op-amp works on the principle of feedback, where feedback network senses output, and develop feedback that will be subtracted from input

               

 •Inverting amplifier

•Non-inverting amplifier

•Differential amplifier





•Integrator/differentiator

•Switched capacitor
-Monolithic integration on CMOS
-Using capacitors instead of resistors
-Dealing with charge
-Suitable for MOS technology

Switched Capacitor

•An inverting amplifier with capacitive load
-Gain is proportional to capacitor ratios
-No DC path

•Adding switches to fix the problem
-Non-overlapping clocks

•In real life
-The gain of op-amp is finite
-Limited effect from parasitic capacitances

Switched Capacitor Integrator

•Building block for monolithic filters
-Frequency response insensitive to parasitic
-Time constant controlled by capacitance ratios than absolute values

Two Two-Stage MOS Op Stage MOS Op-Amp


•Two stages are:
-Differential input
-Gain output
•Performance:

-Input resistance
-Output resistance
-Voltage gain

•Performance:
-Output swing
-Input offset voltage

•Systematic caused by design as we trade off for other performances
•Random caused by mismatch between pieces where they need to be identical

•Performance:

-CMRR

-Common mode input range

-PSRR (low frequency)

Two-stage MOS  Op-Amp with Cascode

•Goal: increasing the voltage gain without frequency degradation
-Using cascode for differential pair to increase unloaded output impedance
-Degrades the input swing

MOS Telescopic Cascode  Op-Amp

•Sometimes just the gain provided by cascode stage is sufficient => drop the second stage in previous topology

•Limitations
-Poor common mode input range

-Limited output swing

-Minimum supply requirements



MOS Folded Cascode Cascode

•Goal: to address the low output swing and limited common mode input range
•Folded since it reverses the signal flow direction

MOS Folded Cascode

•Schematic of a folded cascode Op-Amp
 

http://kaflatooni.yasna.com/op_amp.pdf

http://dahianarosales.blogspot.com/2010/02/operational-amplifiers.html

DAHIANA ALEJANDRA ROSALES HERNÁNDEZ

EES

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