The gain of a transistor amplifier can be adjusted by making the coupling network between two stages a variable attenuator. The attenuator must be designed so that it has negligible effect on the frequency response of the network. The dc operating point of either of the transistors, Fig 19.2, shows two suitable methods of control.
That shown at A is probably the better and is most widely used. It has no effect on the bias setting and has only a slight effect on the low-frequency response. The control shown at B has no effect on the low-frequency response, but will alter the dc voltage across the first transistor.