Consider an NPN transistor circuit. During the positive half-cycle of the signal, the base is positive with respect to the emitter and hence the base-emitter junction is forward biased. This causes a base current and much larger collector current to flow. The positive half-cycle of the signal is amplified in the collector. During the negative half-cycle, the base-emitter junction is reverse biased and hence no current flows. No output flows during the negative half-cycle of the signal. Thus the positive-only amplified output is unfaithful.
A sufficient battery source in the base circuit keeps the input circuit forward biased even during the peak of the negative half-cycle. When no signal is applied, a DC current I C will flow in the collector circuit due to the battery. This is known as zero signal collector current.[1]
The value of zero signal collector current should be at least equal to the maximum collector current due to AC signal alone.
References
edit- ^ Principles of electronics by V K Mehta