A BJT class-AB power amplifier is used as the output stage in a car audio system
ID: 2248815 • Letter: A
Question
A BJT class-AB power amplifier is used as the output stage in a car audio system. A representative circuit is shown in the above figure. Assume that the input signal is large enough to drive both Qn and Qp fully into saturation. We consider four different ways of obtaining significant output power in the questions below.
A. For the first case, VCC is the car battery voltage of 13.6 VDC and VEE is ground. Find the average power that can be delivered to the 4 speaker when the output signal is a sinusoid at maximum amplitude. Calculate a size for capacitor CC for a lower 3dB frequency of 10 Hz.
B. Find the average power that can be delivered to the 4 speaker when the output signal is a sinusoid at maximum amplitude for VCC = +13.6 V and VEE = 13.6 V. Can the coupling capacitor be eliminated? Why?
C. If we connect two of these amplifiers in a bridge arrangement as shown and power each amplifier from VCC = +13.6 V and VEE = ground, find the average power that can be delivered to the 4 speaker when the output signal is a sinusoid at maximum amplitude. Is a coupling capacitor required? Why?
D. In the bridge circuit shown above, what value of VCC is needed to deliver 200 W to the speaker?
BIAS BJT Parameters: 17 Vo very large CE (sat)0 V 0- 4Explanation / Answer
A)
Vcc = 13.6V,
VE = 0 V
RL = 4 ohm,
f = 10 Hz,
Vo= Vcc - Vce = 13.6 - Vce
since Beta is verry large, Vce approimates to 0.
so ,
Vo = 13.6V
Pavg = Vo2/RL = 13.62/4 = 46.24 W
E = 1/2 CV2 ; C = 2E/V2 ; C = 2 * 46.24/13.62 ; C = 0.5 F;
B)
Vcc = 13.6V
VEE = -13.6V
RL = 4 Ohm
Vo = 13.6 - (-13.6) = 27.2V
Pavg = 27.22/4 = 184.96Watt;
In This case if freuency goes at a very high level then the Capacitance will get shorted.
C)
Vcc = 13.6V
Vee = 0V
Pavg = (Vo1 - Vo2)2/RL = 46.24 Watt
The coupliing capacitor will not be required as the op- amp will be able to filter out the noise.
D)
The value of Vcc should be 28.28 V = square root of (800) to deliver 200 W to the speaker.
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