Amplifiers

Impedance Matching Line Driver

Impedance matching line driver.

This circuit provides a means of impedance matching between a frequency filter circuit and a low impedance coaxial cable.

The input stage of this circuit is an emitter-follower consisting of transistor Q₁, and represents a high input impedance. The output stage is a complementary-symmetry emitter circuit consisting of transistors Q₂ and Q₃, which are capable of driving a low impedance coaxial cable (90 ohms) terminated in its characteristic impedance without noticeable distortion. Diodes D₁ and D₂ provide bias for transistors Q₂ and Q₃, maintaining a voltage difference between the bases of the transistors to eliminate crossover distortion. Transistor Q₃ conducts on the positive half of the input sine wave and transistor Q₂ conducts on the negative half.