Q2Analog Circuits

In a Class A Amplifier, the transistor is biased such that the operating point (Q-point) lies exactly in the middle of the active region on the AC load line. This ensures the transistor remains forward-biased and conducts current for the full $360^{\circ }$ of the input AC cycle.

Operation: Because it never turns off, it provides highly linear amplification with minimal distortion. However, a significant continuous DC current flows even when no AC input is present, wasting energy as heat.

Efficiency $\eta$ is the ratio of AC output power to DC input power.

1. DC Input Power ($P_{dc}$): $P_{dc}=V_{CC}{I}_{CQ}$

2. AC Output Power ($P_{ac}$): For maximum swing without clipping, peak voltage $V_p=V_{CC}/2$ and peak current $I_p=I_{CQ}$. The RMS values are $V_p/\sqrt{2}$ and $I_p/\sqrt{2}$.

3. Efficiency ($\eta$):

Thus, the maximum theoretical efficiency of a series-fed Class A amplifier is 25%.