Question

If an amplifier working with ±5V DC power supplies, requires a 

minimum output voltage of 3V across a load resistance of 100 ohms, 

calculate the minimum current to be drawn from the power supply, if 

the efficiency of the amplifier is 35%.

Answer 1

$\eta=\frac{U_mI_m}{U_mI_m+\frac{U^2}{R}},\implies$ $I_m=\frac{\eta U^2}{(1-\eta)U_mR},$

$I_m=\frac{0.35\cdot 5^2}{0.65\cdot 3\cdot 100}=44.9~mA.$