# Electronic Devices and Circuits Questions and Answers – Transistor Difference Amplifiers

This set of Electronic Devices and Circuits Multiple Choice Questions & Answers (MCQs) focuses on “Transistor Difference Amplifiers”.

1. What is the voltage gain of a differential amplifier is all amplifier values are equal?
a) 1
b) 0
c) ∞
d) Cannot be determined

Explanation: If all the resistor values are of the equal value, then the circuit will have a voltage gain equal to exactly one or unity. Hence it is also called as a Unity Gain Differential Amplifier. The output voltage expression would be the difference between the two voltages.

2. What is the problem with a single operational difference amplifier?
a) High input resistance
b) Low input resistance
c) Low output resistance
d) High output resistance

Explanation: The problem with a single operational difference amplifier is its low input resistance. This low input resistance results in a loss of a major component of the signal. Operational Amplifier is internally a Differential Amplifier with features like High Input Impedance, Low Output Impedance.

3. What is the total voltage gain of a difference amplifier if the gain of first stage is 4.8 and the gain of the second stage is 1.2?
a) 6.75
b) 6.55
c) 5.76
d) 5.65

Explanation: The total gain of a difference amplifier can be calculated by the formula given below. Where, AV = overall gain, AV1 = voltage gain of first stage and AV2 = voltage gain of second stage.
AV = AV1 × AV2 = 4.8 × 1.2 = 5.76

4. What is the total voltage gain of a difference amplifier if input of first stage is 50V and the output of the second stage is 100V?
a) 20
b) 0.2
c) 2.5
d) 2

Explanation: The total gain of a difference amplifier is the ratio of the output of second stage amplifier to the input of first stage amplifier. Where, Voutput = 100V and Vinput = 50V.
AV = Voutput / Vinput = 100V / 50V = 2

5. How is the voltage gain measured in a difference amplifier?
a) Volts
b) Ampere
c) Decibels
d) Dimensionless

Explanation: The voltage gain is measured in terms of decibels. The total voltage gain is calculated as the product of individual stages or as a sum of all the stages if the gain calculated at each intermediate stage was measured in decibels as well.
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6. How do we calculate the output voltage of a difference amplifier?
a) VO = -Ad × Vd + AC × VC
b) VO = -Ad × Vd – AC × VC
c) VO = Ad × Vd + AC × VC
d) VO = Ad × Vd – AC × VC

Explanation: The differential gain of a difference amplifier is defined as the gain obtained at the output signal with respect to the difference in the input signals applied. To calculate the output voltage of a difference amplifier we use Ad × Vd + AC × VC. Where, Ad is the differential gain and AC and VC represent the common mode gain.

7. What is the purpose of using a difference amplifier?
a) Increase voltage gain
b) Decrease voltage gain
c) Increase current gain
d) Decrease current gain

Explanation: A circuit having a single transistor configuration does not provide suitable bandwidth or gain. The purpose of a difference amplifier is to provide an increase in the voltage gain. The total gain of a difference amplifier is the product of the voltage gains of the discrete stages.

8. How many methods of coupling are used for a difference amplifier?
a) 3
b) 4
c) 2
d) 5

Explanation: There are four types of coupling mechanisms used. They are Resistance – capacitance coupling, thermal coupling, impedance coupling and transformer coupling. Resistance – capacitance coupling is the most vastly used mechanism.

9. What is the total voltage gain of a difference amplifier if the gain of first stage is 10dB, gain of the second stage is 20dB and gain of third stage is 30dB?
a) 60.32dB
b) 50dB
c) 60dB
d) 50.32dB

Explanation: The total gain of a difference amplifier is the sum of the multiple stages when the gain is calculated is decibels. Given, AV1 = 10dB, AV2 = 20dB and AV3 = 30dB
AV = AV1 + AV2 + AV3 = 10dB + 20dB + 30dB = 60dB

10. The coupling device is essential for a difference amplifier.
a) True
b) False

Explanation: One of the main functions of the coupling device in a difference amplifier is to block the direct current from passing through to the input of next stage from the output of the first stage. Another function of the coupling device is to transfer the current from the output of first stage to the input of the second stage.

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