Deck 11: Power Amplifiers

Question 1

A class A amplifier has an 8 Vpp output that is being applied to a 200 Ω load. What is the total ac load power?

A) 320 mW
B) 640 mW
C) 40 mW
D) 80 mW

Answer 1

40 mW

Question 2

The circuit in Figure has the following values: VCC = 12 V, R1 = 48 kΩ, R2 = 15 kΩ, RC = 4.7 kΩ, RE = 2.2 kΩ, RL = 12 kΩ, and hfe = hFE = 200 How much power is the circuit drawing from the dc power source?

A) 2.3 mW
B) 11.7 mW
C) 14.1 mW
D) 457 μW

Answer 2

14.1 mW

Question 3

An amplifier has values of 2ICQrC = 4.44 V and 2VCEQ = 7 V. If the ac output voltage exceeds 7 V, the amplifier will experience

A) saturation clipping.
B) cutoff clipping.
C) both saturation and cutoff clipping.
D) none of the above.

Answer 3

both saturation and cutoff clipping.

Question 4

An amplifier has values of 2ICQrC = 8.6 V and 2VCEQ = 12 V. If the ac output voltage exceeds 8.6 Vpp, the amplifier will experience

A) saturation clipping.
B) cutoff clipping.
C) both saturation and cutoff clipping.
D) None of the above.

Question 5

The circuit in Figure has the following values: VCC = 12 V, R1 = 48 kΩ, R2 = 15 kΩ, RC = 4.7 kΩ, RE = 2.2 kΩ, RL = 12 kΩ, and hfe = hFE = 200 What is the ac saturation current for this circuit?

A) 981 μA
B) 1.74 mA
C) 2.53 mA
D) It cannot be determined with the information given.

Question 6

A transformer-coupled class A amplifier with a transformer turns ratio of 4:1 and a value of RL = 25 Ω draws 220 mW from its dc power supply. What is the efficiency of the circuit?

A) 20.5%
B) 27.3%
C) 40.9%
D) 73%

Question 7

The circuit in Figure has the following values: VCC = 12 V, R1 = 48 kΩ, R2 = 15 kΩ, RC = 4.7 kΩ, RE = 2.2 kΩ, RL = 12 kΩ, and hfe = hFE = 200 What is the ac cut off voltage for this circuit?

A) 6 V
B) 8.5 V
C) 12 V
D) It cannot be determined with the information given.

Question 8

A circuit like the one in Figure has values: I1 = 1 mA, ICQ = 50mA, VCC = 15 V, PP = 9.3 V, and RL = 680 Ω
What is the maximum efficiency of the amplifier?

A) 16.62%
B) 15.9%
C) 20.8%
D) 25%

Question 9

Class C amplifiers usually contain

A) two transistors.
B) an LC tank circuit in the BJT collector circuit.
C) a single BJT that conducts through 360° of the ac input cycle.
D) a single BJT that conducts through 270° of the ac input cycle.

Question 10

A circuit like the one in Figure has values: VCC = 10 V, I1 = 450 μA, and ICQ = 10.55 mA How much power is the circuit drawing from the dc power supply?

A) 4.5 mW
B) 1.21 mW
C) 110 mW
D) Cannot be determined from the information given.

Question 11

Which of the following is true?

A) Efficiency is the ratio of dc input power to ac output power.
B) The power that an amplifier delivers to its equals the difference between its dc input power and the power that the circuit dissipates.
C) Power amplifiers are typically used to drive high impedance loads.
D) None of the above statements are true.

Question 12

The circuit in Figure has the following values: VCC = 12 V, R1 = 48 kΩ, R2 = 15 kΩ, RC = 4.7 kΩ, RE = 2.2 kΩ, RL = 12 kΩ, and hfe = hFE = 200.
What is the compliance of this circuit?

A) 6.6 V
B) 10.5 V
C) 12 V
D) It cannot be determined with the information given.

Question 13

The transformer-coupled class A amplifier has the lowest efficiency rating of all the class A amplifiers.

Question 14

The maximum theoretical efficiency of an RC-coupled class A amplifier is

A) 25%.
B) 50%.
C) 78.5%.
D) 99%.

Question 15

Class B amplifiers usually contain

A) two transistors.
B) an LC tank circuit in the BJT collector circuit.
C) a single BJT that conducts through 360° of the ac input cycle.
D) a single BJT that conducts through 270° of the ac input cycle.

Question 16

The maximum value of VCE in a transformer-coupled class A amplifier will be greater than VCC. This is caused by

A) the input biasing network.
B) the efficiency characteristics of the amplifier.
C) the counter emf produced by the transformer primary.
D) the natural relationship between VCE and VCC.

Question 17

A transformer-coupled class A amplifier has a transformer with a 3:1 turns ratio and values of RL = 100 Ω, VCEQ = 6 V, and ICQ = 8 mA. What is the value of vceoff) for the circuit?

A) 12 V
B) 6 V
C) 13.2 V
D) Cannot be determined from the information given.

Question 18

The circuit in Figure has the following values: VCC = 12 V, R1 = 48 kΩ, R2 = 15 kΩ, RC = 4.7 kΩ, RE = 2.2 kΩ, RL = 12 kΩ, and hfe = hFE = 200 What is the approximate value of power dissipation for the transistor?

A) 5.1 mW
B) 7.4 mW
C) 12 mW
D) Cannot be determined without input signal

Question 19

A transformer-coupled class A amplifier has a transformer turns ratio of 4:1 and a load resistance of 25 Ω. The peak-to-peak value of VCE is 12 V. What is the approximate load power for the circuit?

A) 45 mW
B) 160 mW
C) 90 mW
D) 60 mW

Question 20

A class A amplifier has a compliance of 6 Vpp and a load resistance of 120 Ω. What is the maximum theoretical load power for the circuit?

A) 37.5 mW
B) 300 mW
C) 50 mW
D) 100 mW

Question 21

A given transistor has a power derating factor of 1.8mW/°C and a power dissipation rating of 400 mW at 25°C. How much power can the device dissipate at 120°C?

A) 184 mW
B) 216 mW
C) 229 mW
D) 355 mW

Question 22

A class AB amplifier has values of VCC = 12 V and RL = 150 Ω. What is the maximum theoretical ac load power for the circuit?

A) 120 mW
B) 240 mW
C) 960 mW
D) Cannot be determined from the information given.

Question 23

A class B amplifier has a maximum theoretical efficiency of

A) 25%.
B) 78.5%.
C) 50%.
D) 99%.

Question 24

When properly used, diode bias will prevent

A) crossover distortion and thermal runaway.
B) crossover distortion and saturation clipping.
C) nonlinear distortion and thermal runaway.
D) nonlinear distortion and saturation clipping.

Question 25

A class AB amplifier has a value of VCC = 15 Vdc. The value of VCEoff) for either transistor is approximately equal to

A) 0.7 V.
B) 5 V.
C) 7.5 V.
D) 15 V.

Question 26

The circuit in Figure 11-2 has the following values: VCC = 16 V, R1 = R2 = 470 Ω, RL = 8 Ω, and hfe = hFE = 200
Determine the values of ICsat) and VCEoff) for the transistors.

A) 1 A, 8 V
B) 2 A, 8 V
C) 1 A, 16 V
D) 2 A, 16 V
Chapter 11 Power Amplifiers Answer Key

Question 27

Complementary-symmetry amplifiers are generally preferred over standard push-pull amplifiers because

A) they use complementary transistors.
B) they do not require the use of an output transformer.
C) they have high efficiency ratings.
D) they can drive lower impedance loads.

Question 28

When troubleshooting a class AB amplifier, you determine that one of the two transistors is faulty. At this point, you should

A) test each of the transistors with an ohmmeter.
B) simply replace both transistors.
C) look for physical signs of damage to the two transistors.
D) test each transistor with a transistor checker.

Question 29

Crossover distortion in class B amplifiers is prevented by

A) biasing the transistors deeply into cutoff.
B) biasing the transistors slightly above cutoff.
C) using complementary-symmetry transistors.
D) increasing the load resistance.

Question 30

The circuit in Figure 11-2 is an example of a amplifier.

A) Class A
B) Class B
C) Class C
D) Class AB

Question 31

Heat sink compound may be used to provide both a thermal connection and a mechanical connection. Figure 11-2