Maximum Efficiency for Class B Power Amplifier

Maximum Efficiency for Class B Power Amplifier

We have already seen in our previous article about class B  power amplifier that a push-pull circuit uses two transistors working in class B operation .

For class B operation, the Q – point is located at cut-off on both d.c. and a.c. load lines .

For maximum signal operation, the two transistors in class B amplifier are alternately driven from cut-off to saturation . This is shown in fig.1 .

Fig.1

It is clear that a.c. output voltage has a peak value of  V_CE  and a.c. output current has a peak value of I_C(sat) .

The same information is conveyed through the a.c. load line for the circuit as shown in fig.2 .

Fig. 2

Since the two transistors are identical, half the supply voltage is dropped across each transistor’s collector-emitter terminals.

Maximum average a.c. output  power P_o(max)  is

P_o(max)  = Product of r.m.s. values of a.c. output voltage and a.c. output current

The input d.c. power from the supply V_CC is

Where I_dc  is the average current drawn from the supply V_CC .

Since, the transistor is on for alternating half-cycles, it effectively acts as a half-wave rectifier .

Thus, the maximum collector efficiency of class B power amplifier is 78.5% .