Impedance coupling in a transistor amplifier is very similar to RC-coupling. The difference is the use of an impedance device (a coil) to replace the load resistor of the first stage.
The illustration shows an impedance coupling network between two stages of amplification. L1 is the load for Q1 and develops the output signal of the first stage.
Since the d.c. resistance of a coil is low, the efficiency of the amplifier stage is increased. The amount of signal developed in the output of the stage depends on the inductive reactance of L1. Remember the formula for inductive reactance:
Impedance coupled transistor amplifier.
The formula used below shows that for inductive reactance to be large, either inductance or frequency or both must be high. Therefore, load inductors should have relatively large amounts of inductance and are most effective at high frequencies. This explains why impedance coupling is usually not used for audioamplifiers.
The rest of the coupling network (C1 and R1) functions just as their
counterparts (C1 and R2) in the RC-coupling network. C1 couples the
signal between stages while blocking the d.c. and R1 develops the input
signal to the second stage (Q2).