martes, 16 de marzo de 2010

Operational amplifier comparator circuit

Operational amplifier comparator circuit

Comparator circuits find a number of applications in electronics. As the name implies they are used to compare two voltages. When one is higher than the other the comparator circuit output is in one state, and when the input conditions are reversed, then the comparator output switches.
These circuits find many uses as detectors. They are often used to sense voltages. For example they could have a reference voltage on one input, and a voltage that is being detected on another. While the detected voltage is above the reference the output of the comparator will be in one state. If the detected voltage falls below the reference then it will change the state of the comparator, and this could be used to flag the condition. This is but one example of many for which comparators can be used.
In operation the op amp goes into positive or negative saturation dependent upon the input voltages. As the gain of the operational amplifier will generally exceed 100 000 the output will run into saturation when the inputs are only fractions of a millivolt apart.
Although op amps are widely used as comparator, special comparator chips are often used. These integrated circuits offer very fast switching times, well above those offered by most op-amps that are intended for more linear applications. Typical slew rates are in the region of several thousand volts per microsecond, although more often figures of propagation delay are quoted.
A typical comparator circuit will have one of the inputs held at a given voltage. This may often be a potential divider from a supply or reference source. The other input is taken to the point to be sensed.

Circuit for a basic operational amplifier comparator

There are a number of points to remember when using comparator circuits. As there is no feedback the two inputs to the circuit will be at different voltages. Accordingly it is necessary to ensure that the maximum differential input is not exceeded. Again as a result of the lack of feedback the load will change. Particularly as the circuit changes there will be a small increase in the input current. For most circuits this will not be a problem, but if the source impedance is high it may lead to a few unusual responses.
The main problem with this circuit is that new the changeover point, even small amounts of noise will cause the output to switch back and forth. Thus near the changeover point there may be several transitions at the output and this may give rise to problems elsewhere in the overall circuit. The solution to this is to use a Schmitt Trigger.

In electronics, a Schmitt trigger is a comparator circuit that incorporates positive feedback.
In the non-inverting configuration, when the input is higher than a certain chosen threshold, the output is high; when the input is below a different (lower) chosen threshold, the output is low; when the input is between the two, the output retains its value. The trigger is so named because the output retains its value until the input changes sufficiently to trigger a change. This dual threshold action is called hysteresis, and implies that the Schmitt trigger has some memory. In fact, the Schmitt trigger is a bistable multivibrator.
Schmitt trigger devices are typically used in open loop configurations for noise immunity and closed loop positive feedback configurations to implement multivibrators.
The Schmitt trigger was invented by US scientist Otto H. Schmitt in 1934 while he was still a graduate student, later described in his doctoral dissertation (1937) as a "thermionic trigger". It was a direct result of Schmitt's study of the neural impulse propagation in squid nerves.
The symbol for Schmitt triggers in circuit diagrams is a triangle with an inverting or non-inverting hysteresis symbol. The symbol depicts the corresponding ideal hysteresis curve.

A Schmitt trigger can be implemented with a simple tunnel diode, a diode with an "N"-shaped current–voltage characteristic in the first quadrant. An oscillating input will cause the diode to move from one rising leg of the "N" to the other and back again as the input crosses the rising and falling switching thresholds. However, the performance of this Schmitt trigger can be improved with transistor-based devices that make explicit use of positive feedback to implement the switching.

Schmitt trigger with two transistors
In the positive-feedback configuration used in the implementation of a Schmitt trigger, most of the complexity of the comparator's own implementation is unused. Hence, it can be replaced with two cross-coupled transistors (the transistors that would otherwise implement the input stage of the comparator). An example of such a 2-transistor-based configuration is shown below. The chain RK1 R1 R2 sets the base voltage for transistor T2. This divider, however, is affected by transistor T1, providing higher voltage if T1 is open. Hence the threshold voltage for switching between the states depends on the present state of the trigger.

For NPN transistors as shown, when the input voltage is well below the shared emitter voltage, T1 does not conduct. The base voltage of transistor T2 is determined by the mentioned divider. Due to negative feedback, the voltage at the shared emitters must be almost as high as that set by the divider so that T2 is conducting, and the trigger output is in the low state. T1 will conduct when the input voltage (T1 base voltage) rises slightly above the voltage across resistor RE (emitter voltage). When T1 begins to conduct, T2 ceases to conduct, because the voltage divider now provides lower T2 base voltage while the emitter voltage does not drop because T1 is now drawing current across RE. With T2 now not conducting the trigger has transitioned to the high state.
With the trigger now in the high state, if the input voltage lowers enough, the current through T1 reduces, lowering the shared emitter voltage and raising the base voltage for T2. As T2 begins to conduct, the voltage across RE rises, further reducing the T1 base-emitter potential and T1 ceases to conduct.
In the high state, the output voltage is close to V+, but in the low state it is still well above V−. This may not be low enough to be a "logical zero " for digital circuits. This may require additional amplifiers following the trigger circuit.
The circuit can be simplified: R1 can be omitted, connecting the T2 base directly to the T1 collector, and the connection of the T2 base to V- via R2 can be completely omitted. When T1 conducts, it connects the T2 base to the T2 emitter so that T2 does not conduct. When T1 does not conduct, RK1 pulls up the T2 base and T2 conducts.

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