So far, we have looked at op-amp principles in part 1, linear applications in part 2, and now, we will look at the final part. And this time, we will talk about non-linear applications.
Linear applications are where the output is related to the input in a straight line sort of way as in an audio amplifier. A non-linear application, on the other hand, is where the output follows the input on some slope or curve, such as a logarithmic amplifier. It produces a pulse or change-over in response to a linear condition on the input, such as a Schmidt trigger zero-crossing detector.
If you need some refresher on waveform generators, see the end of this article for links.
Shown below is the simple comparator.
This circuit forms the basis for many applications, including zero-crossing detectors, relaxation oscillators, level shifters, analog-to-digital converter, window detector, and Schmidt trigger, to name a few.
In the figure below, the two R’s are equal and at Vcc/2. When the input exceeds this point, the output rapidly goes high, and when the input falls, it goes low again.
Adding some positive feedback with R2 turns it into a Schmidt trigger with the hysteresis controlled with R2/R1. Most 555 applications are based on the use of their own internal comparator.
Rectifiers and Detectors
The humble diode has great properties when used for power supplies but less so in small signal audio applications. The reason is the 0.6V forward volt drop Vf, which is negligible in proportion to the large AC voltages in PSUs, but significantly large compared to small signal levels. Enter the op-amp. By putting the diode inside the feedback path, we can reduce the apparent Vf by the substantial open-loop gain of the op-amp to practically nothing.
Shown below is a precision rectifier. D1 rectifies and inverts the input AC wave-form resulting in the wave-form shown above R4. IC2 is an adder and inverter and created the full-wave rectified output. Note that putting a capacitor across R2 would result in clean DC.
Shown below is a peak detector where the output follows the peak value of the input. IC1 is doing the detection, and IC2 is just a buffer to preserve a very high impedance needed by the capacitor, which is charging up to the peak value. R3 is slowly discharging it. C1 should be a tantalum type. Such a circuit is often used in an audio amp or mixer to set the input level below the point where clipping might occur.
Constant Volume Amplifier (VCA)
Shown below is a favorite of mine.
The output level remains constant over a large range (about 30dB) of an input signal, and minimal distortion is noticed. Below the circuit is a chart of the measured response. R1 is chosen to set the gain depending on the expected range of input signals and would be:
10k for 50uV-50mV
100k for 500uV-500mV
1M for 5mv-5V
Some other non-linear op-amp applications might include voltage-controlled amplifiers, limiters, and clippers, log amplifiers.
This concludes the three-part series on operational amplifiers. But here are some other helpful resources related to op-amps:
Article on wave-forms: