Level Control and Special Circuits
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. In every case distortion and noise can be traded off by altering the operating level, but in some techniques the non-linearity is so great that a compromise that meets modern performance standards is simply not possible.
A Brief History
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 ...