ANALOGUE ELECTRONICS I

The output signal varies for a full 360° of the input signal. The figure below shows that this requires the Q - point to be biased at a level so that at least half the signal swing of the output may vary up and down without going to a high enough voltage to be limited by the supply voltage level or too low to approach the lower supply level, or 0 V in this description.   This means that the amplifier is biased such that it always operates in the linear region (active region) where the output signal is an amplified replica of the input signal.

In a small-signal amplifier, the ac signal moves over a small percentage of the total ac load line. When the output signal is larger and approaches the limits of the ac load line, the amplifier is a large-signal type. Both large-signal and small-signal amplifiers are considered to be class A if they operate in the linear region at all times, as illustrated below.