Bipolar Junction Transistors

Level Shifter

This circuit is an emitter follower with a variable resistor in the emitter circuit to permit adjustment of the DC output level.

Either a signal or a DC voltage applied at the input is fed through R₁ to the base of Q₁. The emitter of Q₁ is coupled through the variable resistor R₂ to the +18V supply. The output from the arm of this pot can be adjusted both negative and positive with respect to ground with a minimum effect on the signal as long as Q₁ is conducting.

R₃, C₁, and R₄, C₂ form decoupling networks to prevent signal variations from being fed between the circuits through the supplies.