Quite often the single-transistor amplifier discussed in the previous lectures does not provide enough gain for an application, or more often, it does not combine gain with the desired input and output impedance characteristics. Perhaps the most obvious solution (but generally not the best) is to connect several single-transistor amplifiers, or stages, in tandem one after the other. Because of DC biasing considerations, it is usually not practical to connect the output of one stage directly to the input of another; some kind of coupling device must be used that permits a change in the DC level between two stages.
A possible circuit features capacitor coupling between single-transistor stages. The interstage coupling capacitor allows the following transistor to be DC biased without concern for the DC component of the driving signal. Thus the DC analysis of the two transistor stages is separated into two distinct problems.