ABSTRACT
This chapter deals with amplifier stages that are required to have a variable gain, usually over a wide range of +20 dB, +30 dB, or more. These amplifier stages are very common in audio electronics. A typical application would be a guitar amplifier input which has to cover a range of playing styles and pickup sensitivities. In many cases a minimum gain of unity (0 dB) is quite acceptable, and this simplifies matters considerably. A series-feedback amplifier configuration can be used, which inherently has a minimum gain of unity and can also provide a high or very high input impedance. Combining one stage with another so that less total parts are needed to perform two functions is certainly good practice economically, but rarely done unless really necessary because it is usually difficult to design a stage so that the two functions do not interact or interfere with each other.
