QUIZ 2 – Open Book.
QUESTION
23 September 2025
1
Consider an ideal class B output stage amplifier. This amplifier must deliver an average
power of 80W to a 4 ohm Speaker. The peak output voltage must not be larger than 80% of
the supply voltage VCC.
a) Draw the approximate and idealised circuit diagrams of the class B amplifier. Clearly
show which is the approximate and which is the idealised schematic
(6)
Idealistic
Approvimate
b) Explain the functioning of a class B amplifier using the approximate diagram used in
A
(4)
-
The upper NPN conducts for the positive half-cycle: it sources current into RL so
Vout > 0.
The lower PNP conducts for the negative half-cycle: it sinks current from RL so
Vout < 0.
Each device conducts for ~1800 (Class-B), giving near-zero quiescent current,
high efficiency.
With real VBE thresholds, there’s a small crossover region near 0 V where
neither device conducts (the source of crossover distortion in pure Class-B)
c) Calculate the required VCC of the amplifier
(3)
VCC = 25/0.8 = 31.6V
d) Calculate the peak current in each transistor
(2)
e) Find the average power dissipated in each transistor
(3|)
f) Determine the power conversion efficiency of the circuit
(3)
QUESTION
2
(9)
(8)