Question 12.8: For a class B amplifier using a supply of V CC = 30 V and dr...
For a class B amplifier using a supply of V_{C C}=30 V and driving a load of 16 Ω, determine the maximum input power, output power, and transistor dissipation.
The maximum output power is
maximum P_{o}( ac )=\frac{V_{C C}^{2}}{2 R_{L}}=\frac{(30 V )^{2}}{2(16 \Omega)}= 2 8 . 1 2 5 W
The maximum input power drawn from the voltage supply is
maximum P_{i}( dc )=\frac{2 V_{C C}^{2}}{\pi R L}=\frac{2(30 V )^{2}}{\pi(16 \Omega)}= 3 5 . 8 1 W
The circuit efficiency is then
maximum \% \eta=\frac{P_{o}( ac )}{P_{i}( dc )} \times 100 \%=\frac{28.125 W }{35.81 W } \times 100 \%=78.54 \%
as expected. The maximum power dissipated by each transistor is
\operatorname{maximum} P_{Q}=\frac{\text { maximum } P_{2 Q}}{2}=0.5\left(\frac{2 V_{C C}^{2}}{\pi^{2} R_{L}}\right)=0.5\left[\frac{2(30 V )^{2}}{\pi^{2} 16 \Omega}\right]= 5 . 7 WUnder maximum conditions a pair of transistors each handling 5.7 W at most can deliver 28.125 W to a 16-Ω load while drawing 35.81 W from the supply.