ABSTRACT
A circuit block that gives voltage-control of gain is a difficult thing to implement. The basic function is simply that of multiplication, but most of the practical techniques introduce significant distortion. The difficulty of implementing a good voltage-variable gain element is testified to by the number of technologies that have been tried. A variable attenuator could be made by varying the current flow through diodes, which varied their effective resistance; the control signal inevitably got mixed up with the audio signal, and linearity was poor. The introduction of junction field-effect transistors (JFETs) as gain control devices was a great advance. These devices promised good isolation between control-voltage and signal, instantaneous operation, and lower distortion than existing methods. With the JFET conducting, the major non-linearity is second-harmonic distortion, which can be much reduced by adding half the drain-source voltage to the gate control voltage. Voltage-controlled amplifier is the name given to a specific kind of variable gain device.
