Bipolar Junction Transistors
This circuit is an emitter follower with a variable gain input and an adjustable output DC level.
The input signal is developed across R1 to ground. Since the emitter follower (Q1) has high input impedance, the amount of signal fed to its base through the current limiting resistor R2 will be proportional to the setting of the arm of R1. As the arm of R1 is moved closer to the input, the signal fed into the emitter follower is increased.
The emitter of Q1 is connected through the level pot R3 and the resistor R4 to the -18V supply. As the level pot is adjusted, the DC that appears at output can be varied with a minimum effect on the signal amplitude.