In this chapter, you have examined a variety of non-linear op amp circuits. They are deemed non-linear because their input/output characteristic is not a straight line.
Precision rectifiers use negative feedback to compensate for the forward drop of diodes. By doing so, very small signals may be successfully rectified. Both halfwave and full-wave versions are available. An extension of the precision rectifier is the precision peak detector. This circuit uses a capacitor to effectively lengthen the duration of peaks. It can also be used as an envelope detector.
Other active diode circuits include clampers and limiters. Clampers adjust the DC level of an input signal so that the output is either always positive or always negative. The furthest peaks will just hit 0 V. Clampers may also be used with a DC offset for greater control of the output signal. Limiters force their output to be no greater than a preset value. It may be thought of as a programmable signal clipper.
Function generation circuits are used to create piece-wise linear approximations of transfer functions. These can be used to compensate for the non-linearities of transducers and measurement devices. There are two basic approaches for realizing a design: (1) using Zener diodes, and (2) using a biased-signal diode network. The Zener scheme is a bit easier to set up, but does not offer the performance of the biased-diode form.
In order to improve the performance of comparators, positive feedback may be used. The resulting circuits are called Schmitt Triggers and have much greater noise immunity than simple open loop op amp comparators do. Specialized comparators are also available. These devices are generally faster than simple op amps, and offer more flexibility for logic family interfacing.
Log and anti-log amplifiers utilize a transistor in their feedback loops. The result is a form of signal compression for the log amp, and an inverse signal expansion for the anti-log amp. Specialized log/anti-log ICs are available that take much of the tedium out of amplifier design and testing.
Finally, a four-quadrant multiplier produces an output that is proportional to the product of its two inputs. These devices may be used for a number of applications, including balanced modulation, frequency doubling, and more.
In short, although op amps are linear devices, they may be used with other components to form non-linear circuits.