Coupling methods
In all the amplifiers you have seen so far, with the exception of push-pull circuit, capacitors have been used to allow alternating current to pass while blocking direct current. But there is also other way to do this, and in some amplifier systems, it is preferred. This is use of a transformer to couple the signals from one stage to next stage.
An example of transformer coupling is shown in the figure given below. Capacitors C1 and C2 keep one end of transformer primary and secondary at signal ground. Resistor R1.
Figure--At point A, frequency response for music; at point B, for voice signals; at point C, for narrowband digital signals.
limits the current through the first transistor, Q1. (In some cases, R1 can be eliminated.) Resistors R2 and R3 give the proper base bias for transistor Q2.
The basic disadvantage of this scheme is that it costs larger than capacitive coupling. But transformer coupling can give an optimum signal transfer between amplifier stages having the minimum of loss. This is due to the impedance matching ability of transformers. The turns ratio of a transformer affects not the input and output voltage only, but the ratio of impedances. By selecting right transformer, output impedance of Q1 can be exactly matched to input impedance of Q2.
Figure-- A simple volume control
Figure-- Transformer coupling.
In some amplifier systems, capacitors are added across the primary and/or secondary of the transformer. It results in the resonance at a frequency which is determined by the capacitance and the transformer winding inductance. If set of amplifiers is intended for one frequency (and this is the case in RF systems), this method of coupling, called as tuned-circuit coupling, enhances system efficiency. But care should be taken to be sure that amplifier chain does not get so efficient that it oscillates at resonant frequency of the tuned circuits!