Characteristics of Operational Amplifier MCQ - Objective Question Answer for Operational Amplifier Characteristics Quiz

61. A feedback amplifier is also called as

A. Open-loop amplifier
B. Closed-loop amplifier
C. Feedback network amplifier
D. Looped network amplifier

Answer: B

A feedback amplifier is sometimes referred to as a closed-loop amplifier because the feedback forms a closed loop between input and output.

62. How many types of configurations are available for feedback amplifiers?

A. Six
B. Four
C. Two
D. Eight

Answer: B

There are four types of configuration available. They are voltage series feedback, voltage shunt feedback, Current series feedback, and Current shunt feedback.

63. Which of the following is not a feedback configuration?

A. Current-series feedback
B. Voltage-shunt feedback
C. Current-Voltage feedback
C. Current-Shunt feedback

Answer: C

In a feedback amplifier, either current or voltage can be fed back to the input, but both current and voltage cannot be feedback simultaneously.

64. When load current flows into the feedback circuit, the configuration is said to be?

A. Current-shunt feedback
B. Voltage-shunt feedback
C. Voltage-series feedback
D. All of the mentioned

Answer: B

In the current-series and current-shunt feedback circuit, the load current flows into the feedback circuit.

65. On what criteria does the feedback amplifier are classified?

A. Signal fed back to the input
B. Signal applied to an input
C. Signal fed back to an output
D. None of the mentioned

Answer: D

The feedback amplifiers are classified according to whether the voltage or current is fed back to the input in series or parallel.

66. The closed-loop voltage gain is reciprocal of

A. Voltage gain of an op-amp
B. Gain of the feedback circuit
C. Open loop voltage gain
D. None of the mentioned

Answer: B

Comparing the equation of closed loop voltage gain (A_F) and the gain of the feedback circuit (B.. A_F is reciprocal of B

= > A_F = 1+( R_F/ R₁) ; B = R₁/( R₁+ R_F)

= > B = 1+( R₁/ R_F)

Therefore, A_F = 1/B.

67. Select the specifications that implies the inverting amplifier?

A. V₁ = -3v, V₂ = -4v
B. V₁ = -2v, V₂ = 3v
C. V₁ = 5v, V ₂ = 15v
D. V₁ = 0v, V₂ = 5v

Answer: D

In inverting amplifier, the input is applied to the inverting terminal and the non-inverting terminal is grounded. So,the input applied to inverting amplifier can be V₁ = 0v, V₂ = 5v.

68. Specify the voltage gain of a non-inverting amplifier with feedback amplifier with and without feedback?

A. A = V_o/V_id, A_F = V_f/V_o
B. A = V_f/V_id, A_F = V_o/V_f
C. A = V_o/V_id, A_F = V_o/V_in
D. A = V_f/V_id, A_F = V_f/V_in

Answer: C

The voltage gain of an op-amp with feedback is the open-loop voltage gain, A = V_o /V_id. The voltage gain of op-amp without feedback is the closed-loop voltage gain, A_F = V_o /V_in.

69. If the feedback voltage and the output voltage are given as 10v and 4v. Find the gain of the feedback circuit in the voltage-series feedback amplifier?

A. 2.5v
B. 40v
C. 3v
D. 6.2v

Answer: A

Gain of feedback

feedback voltage = 10 V

output voltage = 4V

B = V_f /V_o

= 10v/4v = 2.5v.

70. How is the difference voltage calculated in a closed-loop non-inverting amplifier?

A. V_id = V_o – V_f
B. V_id = V_in – V_f
C. V_id = V_o – V_in
D. V_id = V_f – V_in

Answer: B

Although the input is given to the non-inverting terminal of an op-amp, the difference voltage is equal to the input voltage minus feedback voltage is

V_id = V_in – V_f.

71. Why the feedback circuit is said to be negative for voltage series feedback amplifier?

A. Feedback voltage is 180^o out of phase with respect to an input voltage
B. Input voltage is 180^o out of phase with respect to a feedback voltage
C. Feedback voltage is in the same phase with respect to an input voltage
D. Input voltage is in the same phase with respect to a feedback voltage

Answer: A

Voltage series feedback amplifiers have different voltages, V_id = V_in-V_f.

Therefore, the feedback voltage always opposes the input voltage and is out of phase by 180^o with respect to the input voltage. Hence, the feedback is said to be negative.

72. Determine the closed-loop voltage gain from the given circuit. (Where gain of op-amp = 10⁵).

A. 1090.9
B. 9821.43
C. 9166.66
D. 10000

Answer: B

The closed loop voltage gain

A_F = {[A*(R₄+R₅)]/[(R_F+R₁+(A*R₁)]}

= [10⁵*(10kΩ+1kΩ)]/[1kΩ+10kΩ+(10⁵*1kΩ)]

= 11 × 10⁸/112000

= > A_F = 9821.43.

73. Express closed-loop voltage gain (A_F) in terms of open-loop gain (A. and feedback circuit gain (B)?

A. A_F = A/AB
B. A_F = 1+ (A/AB)
C. A_F = A/(1+AB)
D. A_F = AB/(1+A)

Answer: C

The closed-loop voltage gain in terms of open-loop gain and feedback circuit gain is expressed as A_F = A/(1+AB)

74. Which factor determines the gain of the voltage series feedback amplifier?

A. Open loop voltage gain
B. Feedback voltage
C. Ratio of two resistors
D. Gain of the feedback circuit

Answer: C

In setting the gain of the voltage series feedback amplifier, the ratio of two resistors is important and not the absolute value of these resistors.

For example: If a gain of 11 is desired, we choose

R₁ = 1kΩ and R₁ = 10kΩ or R₁ = 100Ω and R_F = 1kΩ.

75. For the feedback circuit of the voltage series feedback amplifier, find the feedback voltage for the specifications: R₁ = 1kΩ, R_F = 10kΩ, and V_o = 25v.

A. 12.5v
B. 22v
C. 0.9v
D. 2.3v

Answer: D

The feedback voltage

V_f = (R₁*V_o)/(R₁+R_F)

= (1kΩ*25v)/(1kΩ+10kΩ)

= 2.272v ≅ 2.3v.

76. What must be the value of external components used in the voltage series feedback amplifier?

A. Less than 1MΩ
B. Less than 10MΩ
C. Less than 100MΩ
D. Less than 0.1MΩ

Answer: A

All external component values should be less than 1MΩ that is used in the voltage series feedback amplifier. So, that they do not adversely affect the internal circuitry of the op-amp.

77. Define the input resistance with feedback for the voltage series feedback amplifier?

A. R_IF = (1-AB.
B. R_IF = (AB-1)
C. R_IF = (1+AB.
D. None of the mentioned

Answer: C

In the feedback amplifier, the input resistance of the op-amp with feedback is (1+AB) times that of without feedback.

78. When the non-inverting input terminal of an op-amp is equal to that of the inverting input terminal (ideally)

A. a–>∞
B. V_id ≅ 0
C. A_F = 1+( R_F / R₁)
D. All of the mentioned

Answer: D

When the input at both the terminal are equal, the difference input voltage,

V_id = V_in1– V_in2.

= > V_id≅0 (∵V_in1 = V_in2)

The open loop voltage gain, A = V_o/ V_id = V_o/0 = ∞ and

the closed loop voltage gain, A_F = V_o/ V_in = 1+ (R_F/ R₁).

79. Find the input and output resistance for the circuit shown.
Specification for 741 op-amp : A = 400000 ; R_i = 33MΩ; R_o = 60Ω;
R_F = 11kΩ; R₁ = 2kΩ; Supply voltage = ± 15v; Maximum output voltage swing = ± 13v.

A. R_IF = 66MΩ, R_OF = 30Ω
B. R_IF = 30MΩ, R_OF = 6kΩ
C. R_IF = 15kΩ, R_OF = 50MΩ
D. None of the mentioned

Answer: A

A_F = 1+(R_F/R₁) = 1+(11kΩ/2kΩ) = 6.5;

B = 1/ A_F = 1/6.5 = 0.154;

Input resistance of R_IF = R₁(1+AB. = 33MΩ[1+(6.5*0.154) ] = 66MΩ;

Output resistance of R_OF = R_o/(1+AB. = 60/[1+(6.5*0.154) ] = 29.98 ≅30Ω.

80. The output resistance of the op-amp with feedback is

A. Same as that of the output resistance without feedback
B. Greater than that of the output resistance without feedback
C. Smaller than that of the output resistance without feedback
D. None of the mentioned

Answer: C

The output resistance in a voltage series feedback amplifier is (1/(1+AB.) times the output resistance of the op-amp. Therefore, the output resistance of the op-amp with feedback is much smaller than the output resistance without feedback.

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