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Comparators

What happens to an operational amplifier if the negative feedback is removed? With no feedback and very high gain, obviously the output voltage will go to one extreme limit or the other. Typically this is limited to just outside the ±10 volt limit used in analog computers, and is inherently current-limited to avoid any possible damage. But is there really any use for such a circuit?


An op amp connected as a comparator.

The circuit to the left shows the basic concept here. If Vin is positive, Vout will go full negative. For purposes of discussion, we will assume the output is limited by internal circuitry to ±10 volts, so the output will be -10 volts whenever the input is positive by any significant amount. By the same token, Vout will become +10 volts whenever Vin goes negative.

As shown, this circuit operates as a zero crossing detector. That is, its output changes polarity whenever the input voltage crosses zero to change polarity. In the configuration shown, the output voltage polarity is opposite to the input polarity. However, the two inputs can be swapped, in which case Vout will have the same polarity as Vin.


Constraining the comparator to digital output voltages.

If we re-introduce negative feedback, using a Zener diode such as the 1N751 (rated at 5.1 volts), we can limit the output voltage to the standard digital logic values of +5 volts (logic 1) and 0 volts (logic 0). Since we are using negative feedback here, the op amp must operate in inverting mode. The circuit to the right shows this configuration.

Again, this circuit operates as a zero-crossing detector, producing a True or logic 1 output whenever the input voltage goes negative. As such, it can also operate as a sign detector.

However, this circuit is still limited because it cannot detect any other input voltage than zero. In a wide range of situations, we would like to be able to detect whether or not the input is above (or below) some arbitrarily specified non-zero voltage. As shown, neither of these circuits can do that.



A general comparator circuit.

A more general comparison circuit is shown to the left. This circuit is a true comparator, in that it corrrectly indicates a voltage comparison between a reference voltage (Vref) and an unknown input voltage (Vin) at the other. As with the original zero crossing detector above, the two inputs may be swapped according to the desired sense of the output. The resistor and Zener diode in the output circuit convert the full-range output swing to digital levels, if that is desired. However, these components are not required for the basic voltage comparison.

Because this circuit effectively compares the two input voltages and produces a corresponding output, it is known as a comparator. Comparators find a wide range of applications in practical, commercial circuits. We'll see some useful applications of this basic concept on several of these pages.


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