of the linear part of the transfer curve then the dc base current I B would be constant and corresponding collector current I C will also be constant. The dc voltage V CE = V CC - I C R C would also remain constant. The operating values of V CE and I B determine the operating point, of the amplifier. If a small sinusoidal voltage with amplitude v s is superposed on the dc base bias by connecting the source of that signal in series with the V BB supply, then the base current will have sinusoidal variations superimposed on the value of I B .
As a consequence the collector current also will have sinusoidal variations superimposed on the value of I C , producing in turn corresponding change in the value of V O . We can measure the ac variations across the input and output terminals by blocking the dc voltages by large capacitors. In the discription of the amplifier given above we have not considered any ac signal. In general, amplifiers are used to amplify alternating signals.
Now let us superimpose an ac input signal v i (to be amplified) on the bias V BB (dc) as shown in Fig. . . The output is taken between the collector and the ground.
The working of an amplifier can be easily understood, if we first assume that v i = . Then applying Kirchhoff’s law to the output loop, we get V cc = V CE + I c R L ( . ) Likewise, the input loop gives V BB = V BE + I B R B ( . ) When v i is not zero, we get V BE + v i = V BE + I B R B + I B (R B + r i ) The change in V BE can be related to the input resistance r i [see Eq.
( . )] and the change in I B . Hence v i = I B (R B + r i ) = r I B The change in I B causes