Answer to Question #85270 in Electric Circuits for Anand

Question

Why is it necessary in the case of cascade amplifier to use coupling network between
the two stages? What are the advantages of transformer coupling? Why is it not
preferred at low frequencies?

Answer 1

I'll try to reply with three shorts answers.

1) Because overall quality of a signal for directly connected stages will be very bad. A coupling network allows the following: blocking DC currents (RC coupling), keeping effective load resistance the same (transformer coupling), keeping the required bandwidth at the final stage.

2) Transformer coupling provides higher voltage gain than RC coupling, higher stages' impedance matching, direct current will not be transferred to the next stage, signal power does not decrease in the base resistors or collector.

3) Because the transformer coupling is very ineffective at low frequencies. Low frequency coupling transformers require a lot of wires and material for its magnetic core, thus making the transformers bulky, heavy and expensive.