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The input signal $V_{in}$ shown in the figure is a 1 kHz square-wave voltage that alternates between $+7\,\text{V}$ and $-7\,\text{V}$ with a 50% duty cycle. Both transistors have the same current gain, which is large. The circuit delivers power to the load resistor $R_L$. What is the efficiency of this circuit for the given input? Choose the closest answer.

1. 46%
2. 55%
3. 63%
4. 92%

Correct answer: 92%

Solution

With large transistor current gain, the input power required to drive the bases is negligible compared with the power delivered to the load. Since the output is a square wave, the circuit operates close to an ideal switching amplifier, giving high efficiency. The closest option is 92%.

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