martes, 16 de marzo de 2010

Operational Amplifiers

Operational Amplifiers:
Operational amplifiers (op amps) were originally used for mathematical operations in 'analog' computers. They typically have 2 inputs, a positive (non_inverting) input and a negative (inverting) input. A signal fed into the positive (non_inverting) input will produce an output signal which is in phase with the input. If the signal is fed into the negative (inverting) input, the output will be 180 degrees out of phase when compared to the input.
There are a bazillion (technical term) applications for op amps. The following section is an attempt to give you a basic understanding of just a few applications. None of the power supply connections are shown. Most op amp circuits used in audio use a ±15 volt power supply (especially when the audio equipment has a switching power supply). They can also be used with a single ended supply (no negative voltage) in head units and other such equipment that have no switching power supply.
The diagram below shows the schematic symbol for an op amp.
OP AMP Operation:
The circuit below shows a simple buffer circuit. The input impedance of an op amp is extremely high (on the order of 1012 ohms). It might be used if the input signal to the op amp was coming from a source which could supply almost no current. The output of the op amp can easily drive 1000 ohms or more. The output, when used as a buffer, will theoretically be identical to the input signal. I can't say it is identical because there is a small amount of distortion in all amplifier circuits. The distortion in this circuit would be EXTREMELY low and would most likely be inaudible.
OP AMPs as Amplifiers:
An op amp can also easily amplify a signal such as audio. The diagram below shows the circuit for an op amp that would give an output signal twice as large as the input. Op amps don't like errors. To get amplification, you induce an error in the signal going back to the negative input of the op amp. An op amp will do everything in it's power to get the signal on the negative input to match the signal on its positive input. To get an output that's twice as large as the input, you use 2 equal value resistors as a voltage divider to reduce the return (feedback) signal at the negative input by half. If the return signal doesn't match the input signal, the op amp will increase the output until the signal returned to the negative input is the same as the input to the positive input. Since the voltage divider cuts the signal in half, the signal at the output must be doubled. You can create any amount of gain needed by changing the value of ONE of the resistors in the 'feedback' path. The actual limit of gain will be determined by the op amp design. When using an op amp as a non-inverting amplifier, the gain will always be greater than or equal to 1. To get a gain of less than 1, you need to use a voltage divider on the input signal.
OP AMP Inverters:
As I said earlier, an op amp can produce a signal which is 180 degrees out of phase (inverted) with respect to the input signal. The diagram below shows an op amp used as an inverter. To use an op amp as an inverting amplifier, you must send the signal into the negative input instead of the positive input. As I said before, the op amp will do everything it possibly can to make the voltage (signal) on the the negative input match the positive input. In the following diagram, you can see that the positive input is connected to ground. It's shown as being connected through a resistor but the resistance to ground in unimportant. What is important is that the positive input has no signal (it's connected to the reference, ground). This means that the op amp's negative input will have no visible (voltage) signal on it. When you're driving the negative input it will act as a virtual ground. The input is converted from a voltage drive to a current drive. The change in current is what drives the op amp. This is important to know because when you look at the negative input with an oscilloscope, you will see no signal (when the circuit is an inverting amplifier). I said earlier that the op amp inputs had a very high impedance. While this is true, when using the inverting input with feedback (which is necessary for audio reproduction), the input impedance becomes the value of the input resistor.
OP AMP Error Correction:
An op amp is commonly used in a circuit where error correction is required. Op amps can't (generally) supply a large amount of current at its output. If a signal is fed to the positive input of an op amp and the op amp is driving a circuit which CAN supply a large amount of current, the output of the whole system can be fed back into the negative input of the op amp. This will allow the op amp to compare the output (of the whole system) to the input signal and correct as needed. If the op amp is used in a circuit which needs little current at its output, the op amp can still monitor the output and correct as needed.
The positive input on the op amp is analogous to the green arrow. The height of the green arrow would be analogous to the voltage on the positive input. The sensor would be analogous to the valves which would also be analogous to the negative input of the op amp. The error correction would come at the output of the op amp (instead of the hydraulic actuator).
The next diagram has a resistor in series with the output of the op amp and the load which is to be driven by the op amp. The resistor represents anything that may be between the op amp and the load. The resistor could actually be a long run of wire, resistance in the copper of a printed circuit board or anything else that may cause the signal to be distorted. If the op amp didn't monitor the signal at the load, the signal would be distorted (in this case, the simple series resistance would only reduce the signal level). If the resistor would instead be an external circuit designed to increase the output current (such as the transistors, resistors, capacitors... of a power amplifier), the output of the op amp may not even resemble the final output signal. The op amp would do everything possible to get the final output to match the input signal.


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