Besides their use as an amplification device, op amps may be used for a variety of other purposes. In this chapter, we will look into a number of circuits that fall under the general classification of non-linear applications. The term non-linear is used because the input/output transfer characteristic of the circuit is no longer a straight line. Indeed, the characteristic may take on a wide variety of shapes, depending on the application. It might seem odd to be studying non-linear circuits when op amps are generally considered to be linear devices. Rest assured that non-linear elements are often essential tools in a larger system. The general usage of non-linear elements is that of wave shaping. Wave shaping is the process of reforming signals in a desired fashion. You might think of it as signal sculpting.
Wave shaping runs the gamut from simple half-wave rectification up through transfer-function generation. Although many of the applications presented in this chapter can be realized without op amps, the inclusion of the op amp lends the circuit a higher level of precision and stability. This is not to say that the op amp versions are always superior to a passive counterpart. Due to bandwidth and slew rate limitations, the circuits presented perform best at lower frequencies - primarily in the audio range and below. Also, there are definite power handling and signal level limitations. In spite of these factors, the circuits do see quite a bit of use in the appropriate areas. Many of the applications fall into the instrumentation and measurement areas.
Besides the wave shaping functions, the other uses noted here are comparators and logarithmic amplifiers. General-purpose comparators made from simple op amps were explained in Chapter 2. In this chapter, specialized devices are examined, and their superiority to the basic op amp comparator is noted.
Logarithmic amplifiers have the unique characteristic of compressing an input signal. That is, the range of signal variation is reduced. The mirror image of the log amplifier is the anti-log amplifier. It increases the range of input signal variation.