Series fed Class A amplifier maximum efficiency

A series-fed class A amplifier has the load (RC) connected in series with the transistor from the supply, and the base bias is given through a resistor RB. The transistor conducts for the full 360° of the input cycle.

Circuit diagram

                 +VCC
                  |
          +-------+-------+
          |               |
        [RB]            [RC]  (load)
          |               |
          |               +-----o Vo
          |               |
          |               C
 Vi o--||-+--------------B   (NPN)
                          E
                          |
                         GND

Derivation of efficiency

Input power (dc): Pi = VCC · ICQ (the signal does not change the average current in a class A stage).

Output power (ac), for a sinusoidal swing with peak collector current Ic(p) and peak output voltage Vc(p) = Ic(p) RC:

Po = Vc(p) Ic(p)/2 = Ic(p)² RC/2

Efficiency η = Po/Pi × 100%

Maximum swing: For the largest undistorted swing, the Q point is set at the middle of the load line: VCEQ = VCC/2 and ICQ = VCC/(2RC). The peak current then equals the quiescent current: Ic(p) = ICQ = VCC/(2RC), and Vc(p) = VCC/2.

Po(max) = (VCC/2)(VCC/2RC)/2 = VCC²/(8RC)

Pi = VCC · ICQ = VCC · VCC/(2RC) = VCC²/(2RC)

ηmax = Po/Pi = [VCC²/(8RC)] / [VCC²/(2RC)] = 1/4

Maximum efficiency = 25 %

In general, for a peak-to-peak output swing Vpp: η = 25 (Vpp/VCC)² %, which reaches 25 % when Vpp = VCC.

Practical efficiency is lower, because the signal swing never reaches the full supply. The efficiency is low because a constant dc current is drawn, and power is dissipated in RC and the transistor even when there is no signal.