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

Linear Integrated Circuit MCQ, Electronics MCQ / June 19, 2022

111. Find the value of input resistance for differential amplifier with one op-amp. If R1  = R2 = 100Ω and RF  = R3  = 5kΩ.

A. RIFx  = 110Ω; RIFy  = 6.7kΩ
B. RIFx  = 100Ω; RIFy  = 5.1kΩ
C. RIFx  = 150Ω; RIFy  = 7.2kΩ
D. RIFx  = 190Ω; RIFy  = 9.0kΩ

Answer: B

The input resistance of inverting amplifier is RIFx  = (R1) and the input resistance of non-inverting amplifier is RIFy  = (R2+ R3)

= > ∴ RIFx  = 100Ω and

= > RIFy  = 100+5kΩ = 5.1kΩ.

 

112. What is the net output voltage for a differential amplifier with one op-amp?

A. Vo  = -(RF /R1)*Vx
B. Vo  = -(RF /R1)*(Vx -Vy)
C. Vo  = (1+RF /R1)*(Vx -Vy)
D. None of the mentioned

Answer: B

The net output voltage for a differential amplifier with one op-amp is given as

Vo = -(RF /R1)*(Vx-Vy).

 

113. The characteristics of a non-inverting amplifier are identical to

A. Differential Amplifier with one op-amp
B. Differential Amplifier with two op-amp
C. Differential Amplifier with three op-amp
D. None of the mentioned

Answer: B

The characteristics of a non-inverting amplifier are identical to the differential amplifier with two op-amps as the gain of the amplifier are same. i.e. A = 1+(RF /R1).

 

114. How is it possible to vary the voltage from the closed-loop gain to the open-loop gain?

A. By using a differential amplifier with a larger gain
B. By using a differential amplifier with a small gain
C. By using a differential amplifier with variable gain
D. By using a differential amplifier with differential gain

Answer: C

The variable gain in the differential amplifier can be obtained by using a potentiometer. So, depending on the position of the wiper in the potentiometer, voltage gain can be varied from the closed-loop gain to the open-loop gain.

 

115. A differential amplifier with two op-amps has the following specifications:
R1 = R3 = 1.5kΩ ; RF = R2 = 5.7kΩ. Compute the input resistance for the following circuit.

A. RIFx  = 83GΩ, RIFy  = 317GΩ
B. RIFx  = 90GΩ, RIFy  = 400GΩ
C. RIFx  = 59GΩ, RIFy  = 269GΩ
D. RIFx  = 36GΩ, RIFy  = 156GΩ

Answer: A

The input resistance of the first stage is given as RIFy,

= > RIFy  = Ri*[1+(A *R2)/( R2+ R3)]

= RIFy  = 2MΩ*[(1+200000*5.7kΩ)/(5.7kΩ+1.5kΩ)]

= 3.166*1011  = 317GΩ.

Similarly the input resistance of the second stage amplifier is given as RIFx,

= > RIFx  = Ri*[1+(A *R1)/( R1+ RF)]

= RIFx  = 2MΩ*[(1+200000*1.5kΩ)/(5.7kΩ+1.5kΩ)] = 83GΩ.

 

116. Determine the output voltage from the diagram. Where V1 = -4.3Vpp and V2  = -5.1Vpp sinewave at 1000hz.

A. 12 Vpp sinewave at 1000hz
B. 13.3 Vpp sinewave at 1000hz
C. 14 Vpp sinewave at 1000hz
D. 11 Vpp sinewave at 1000hz

Answer: B

In a differential amplifier with two op-amps, R1 = R3 and RF = R2.

The closed loop voltage gain, AD  = 1+( RF/ R1) = 1+(7.5kΩ/480Ω) = 16.62.

Output voltage , Vo  = AD(V1– V2) = 16.62(-4.3Vpp+5.1Vpp) = 13.29 = 13.3 Vpp sinewave at 1000hz.

 

117. Determine the output resistance of the differential amplifier with three op-amps. The op-amp used is 741c, with A = 200000 and Ro. The output and difference of input voltages are 44 and 11.

A. 5.5mΩ
B. 3.5mΩ
C. 2.4mΩ
D. 1.5mΩ

Answer: D

Gain AD  = Vo/Vid  = 44/11 = 4.

The output resistance of differential amplifier is given as

ROF = Ro/[1+(A/AD)]

= 75/[1+(200000/4)] = 1.5mΩ.

 

118. Find the input resistance for the given circuit

A. RIF  = Ri*[1+(A*R4+R5)/(2* R4+R5)].
B. RIF  = Ri*[1+(A*R4)/(2* R4+R5)].
C. RIF  = Ri*[1+(A*R4+R5)/(R4+R5)].
D. None of the mentioned

Answer: A

The input resistance of the op-amp with feedback is given as RIF = Ri*(1+AB.

From the diagram, B = Vf/Vo  = [R4+R5)/(2* R4+R5)].

Therefore, RIF  = Ri*[1+(A*R4+R5)/(2* R4+R5)].

 

119. The bandwidth of the differential amplifier increases, if the value of

A. Open loop voltage gain decreases
B. Closed-loop voltage gain decreases
C. Differential voltage gain decreases
D. All of the mentioned

Answer: B

The bandwidth of the differential amplifier is inversely proportional to the closed-loop voltage gain.

 

120. Calculate the bandwidth for the given circuit. (Where the 741 op-amp has a gain of 200000).

A. 560hz
B. 390khz
C. 25.6khz
D. 1.5Mhz

Answer: C

Gain of the amplifier

AD  = -{[1+(2*R4/R5)]*(RF/R1)}

= -{[1+(2*5kΩ/2.5kΩ)]*(3.9kΩ/500Ω)} (since, RF  = R3 and R2  = R1).

= > AD  = -5*7.8 = -39.

The bandwidth for the circuit is calculated as

fF  = (A*fo)/AD

= (200000*5hz)/39

(For 741 op-amp 5hz is the break frequency)

= > fF  = 25.6khz.

 

121. Calculate the output voltage. If Vx = 3.9Vpp and Vy  = 5.5 Vpp sinewave at 1khz for the following circuit.

A. 11 Vpp sinewave at 1Khz
B. -8Vpp Vpp sinewave at 1Khz
C. -10 Vpp sinewave at 1Khz
D. 6 Vpp sinewave at 1Khz

Answer: B

Voltage gain, AD = -RF/ R1

= -15kΩ/3kΩ = -5kΩ.

The output voltage, Vo  = – AD*(V x– V y)

= 5*(3.9-5.5) = -8 Vpp sinewave at 1Khz.

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