Class B maximum efficiency and condition

Maximum efficiency of a class B amplifier = π/4 × 100 = 78.5 %.

Condition

It occurs when the peak output voltage equals the supply voltage, Vm = VCC (the full possible swing, ignoring the transistor saturation voltage VCE(sat)). In other words, the peak load current is Im = VCC/RL, so the output swings across the entire supply range.

Reason

For a sinusoidal load current of peak Im:

Po = Im² RL/2, Pi = VCC · 2Im/π

η = Po/Pi = (π/4)(Im RL/VCC) = (π/4)(Vm/VCC)

The efficiency rises in direct proportion to Vm, so it is highest at Vm = VCC:

ηmax = π/4 × 100 % = 78.5 %

At smaller outputs the efficiency is lower; for example, at Vm = VCC/2, η = 39.3 %. In practice VCE(sat) and the dead-zone bias reduce the maximum to about 60 to 70 %.

Note

Maximum efficiency (full swing) is not the same point as maximum transistor dissipation. The dissipation in the two transistors is largest at Vm = 2VCC/π ≈ 0.64 VCC, where P2Q(max) = 2VCC²/(π² RL).