Deck 7: BJT Power Amplifiers

Question 1

Figure 1
Refer to Figure 1. The voltage gain, Av, to the load is approximately

A)11
B)7.5
C)5.4
D)2.0

Answer 1

D

Question 2

The amplifier type that frequently has a resonant circuit load is

A)class- AB
B)class- C
C)class- B
D)class- A

Answer 2

B

Question 3

The amplifier type that has the worst efficiency is

A)class- B
B)class- A
C)class- C
D)class- AB

Answer 3

B

Question 4

In a single- stage class- A amplifier, the quiescent current is the same as the

A)ac load current
B)dc collector current
C)ac collector current
D)bias current

Question 5

Figure 7
Refer to Figure 7. The expected dc emitter voltage for Q1 is

A)0 V
B)9.0 V
C)0.7 V
D)8.3 V

Question 6

A heat sink is used to

A)prevent excessive current
B)reduce the junction temperature
C)reduce ambient temperature
D)all of the above

Question 7

Figure 2 This is modification of Figure 1 (added Q₂, Q₃, and R_L, deletion of C₃ and a changed value of R_E1).
Refer to Figure 2. With no applied signal, the dc power dissipated in the load is approximately

A)1.1 W
B)0
C)5.5 W
D)2.8 W

Question 8

Figure 3
Refer to Figure 3. Note the input is a 5 Vrms sine wave. The ac power in RL is ideally

A)1.56 W
B)3.12 W
C)0.31 W
D)0.62 W

Question 9

A class- A amplifier has an input resistance of 520 ▲ and a load resistance of 26 ▲. Assume a 1 Vrms input produces a 5 Vrms output The power gain is

A)5
B)100
C)50
D)500

Question 10

Figure 2 This is modification of Figure 1 (added Q₂, Q₃, and R_L, deletion of C₃ and a changed value of R_E1).
Refer to Figure 2. If Vin = 1 Vrms, the ac power in RL is approximately

A)2.6 W
B)4.0 W
C)680 mW
D)1.3 W

Question 11

Figure 1
Refer to Figure 1. The input impedance is approximately

A)230 ▲
B)186 ▲
C)125 ▲
D)142 ▲

Question 12

Figure 3
Refer to Figure 3. The value of dc bias current in R1 is

A)43 mA
B)90 mA
C)86 mA
D)48 mA

Question 13

Figure 3
Refer to Figure 3. With no signal in, IC(Q1)is ideally

A)0
B)86 mA
C)43 mA
D)48 mA

Question 14

Assume the Q point on a class- A amplifier is centered on the ac load line. The maximum peak- to- peak output voltage is

A)1/2 VCEQ
B)2 VCEQ
C)1/2 VCC
D)VCEQ

Question 15

Figure 5
Refer to Figure 5. The diode

A)produces class- AB operation
B)compensates for the base- emitter drops of the transistors
C)helps prevent crossover distortion
D)all of the above

Question 16

Figure 7
Refer to Figure 7. The power delivered to the load is

A)0.25 W
B)1 W
C)0.5 W
D)2 W

Question 17

Figure 1
Refer to Figure 1. The power gain, Ap, to the load is approximately

A)14
B)4.5
C)23
D)7.9

Question 18

Figure 3
Refer to Figure 3. This type of bias is called

A)current- mirror biasing
B)push- pull bias
C)diode biasing
D)mid- point bias

Question 19

Figure 5
Refer to Figure 5. The center tap on the output transformer (marked
A)should be connected to

A)-VCC
B)ground
C)+V_CC D)nothing, it is left open

Question 20

Figure 6
Refer to Figure 6. The combination of R1, C1, and the base- emitter diode form a

A)positive clipping circuit
B)clamping circuit
C)negative clipping circuit
D)coupling circuit

Question 21

Figure 2 This is modification of Figure 1 (added Q₂, Q₃, and R_L, deletion of C₃ and a changed value of R_E1).
Refer to Figure 2. Assume Vout has dropped to about 1/2 the expected value. A likely cause is that

A)VCC is set to +15 V
B)C2 is shorted
C)R_L is open
D)C₂ is open

Question 22

Class- C amplifiers are frequently used as audio amplifiers.

Question 23

Figure 1
Refer to Figure 1. The dc collector current is approximately

A)50 mA
B)25 mA
C)120 mA
D)85 mA

Question 24

A class- B amplifier with Darlington transistors can use larger bias resistors.

Question 25

The ratio of Pout to PDC is the efficiency of an amplifier.

Question 26

The Q point of a class- C amplifier is below cutoff.

Question 27

The maximum power from a class- A amplifier is equal to ICQVCEQ.

Question 28

In a class- AB amplifier, the transistors are biased slightly above cutoff to avoid

A)clipping the peaks of signals
B)low efficiency
C)loading problems
D)crossover distortion

Question 29

Figure 6
The transistor in a Class- C amplifier is biased for conduction for

A)360°
B)90°
C)180°
D)much less than 90°

Question 30

In a class- B amplifier, the Q- point is at cutoff.

Question 31

The maximum ideal efficiency for a class- A amplifier is

A)80%
B)40%
C)55%
D)25%

Question 32

Figure 4
Refer to Figure 4. The amplifier represented is

A)class- C
B)class- A
C)class- B

Question 33

A class- A amplifier the quiescent power is dissipated only when a signal is present.

Question 34

Figure 7
Refer to Figure 7. The expected ac emitter voltage for Q1 is

A)4.0 Vrms
B)2.0 Vrms
C)0 V
D)none of the above

Question 35

Figure 2 This is modification of Figure 1 (added Q₂, Q₃, and R_L, deletion of C₃ and a changed value of R_E1).
Refer to Figure 2, which is a modified circuit from Figure 1. The voltage gain, AV, of this circuit is approximately

A)3.3
B)5.6
C)2.0
D)4.3