Comparator circuit using an Op Amp, Operational Amplifier

- a comparator circuit using an operational amplifier is easy to design, although some precautions are required to ensure its correct oepration in some circumstances.

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.

Operational amplifier comparator

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 as described on another page.

By Ian Poole

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