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Class B push-pull power and efficiency, 20 V peak

Short answer

Output power Po = Vp²/(2RL) = 20²/(2×16) = 12.5 W. DC input power Pi = 2VCC Vp/(π RL) = 2×30×20/(π×16) ≈ 23.87 W. Efficiency η = (π/4)(Vp/VCC) = 52.4%.

A Class B push pull amplifier is providing 20 V peak signal (Sine Wave) to a 16 Ω load and power supply is VCC = 30 V. Determine the input power, output power and circuit efficiency.

Given: Class B push-pull, peak output voltage Vp = 20 V, load RL = 16 Ω, VCC = 30 V.

Formulas

  • Peak load current: Im = Vp/RL
  • Output power: Po = Vp²/(2RL)
  • Input (dc) power: Pi = VCC · Idc = 2 VCC Vp/(π RL), because each supply gives an average current Im/π per half-cycle, and the two together give 2Im/π
  • Efficiency: η = Po/Pi × 100 %

Calculation

  • Im = 20/16 = 1.250 A
  • Output power Po = 20² / (2 × 16) = 12.50 W
  • Input power Pi = 2 × 30 × 20 / (π × 16) = 23.87 W
  • Efficiency η = 12.50 / 23.87 = 52.4 %

Check with η = (π/4)(Vp/VCC) = 0.7854 × 20/30 = 52.4 %.

The power lost as heat in the two transistors is Pi − Po = 11.37 W.

Answer: Pi = 23.87 W, Po = 12.50 W, η = 52.4 %.

✓ Answer checked by Claude Opus 5.5 (high effort)

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