Class A Power Amplifiers

CLASS A POWER AMPLIFIERS

A power amplifier is known as class A amplifier if the transistor used in the circuit conducts for full cycle of the input signal. The operating point is selected approximately at the center so that the output current follows the input signal.

The transistor is biased in the active region. The transistor is never driven into its cut-off or saturation region. Therefore, the AC input signal is perfectly centered between the amplifiers upper and lower signal limits. The transistor conducts for entire 360°.

The base current changes sinusoidally above and below the quiescent base current. The output collector current also changes sinusoidally above and below the quiescent current value. They are in phase with each other. Due to this I_C change, V_CE will also change sinusoidally but 180° out of phase. The input signal is amplified faithfully without any distortions.

Since the transistor is operated in active region, the collector current and voltage are high which produces large power which is dissipated as heat. Hence, the efficiency of class A power amplifier is low.

The efficiency of a power amplifier is given by

η = average signal power delivered to load / average power drawn from dc source

In biasing, Q-point is fixed by V_CC & R_B.

Collector emitter voltage,

V_CE = V_CC - I_C R_C

In the load line, Q point is located as shown in Fig.5.6.

The point is set halfway between 0 and V_CC/R_C, and the largest collector current swing is obtained.

The input power is given by

Power delivered to the load by the transistor is

Maximum output power = P_o(ac)(max)

If Q-point is at the middle of loadline, then

Maximum efficiency η (max) 

The power dissipated as heat by the transistor is

Advantages

1. Simple construction

2. Distortionless output voltage

3. Cheaper and less bulky

Disadvantages

1. Very low efficiency (25%)

2. Large power dissipation in the transistors

3. High output impedance vonsibits aldiz og mun