# Differential Amplifiers Viva Questions

## Differential Amplifiers Viva Questions

Differential Amplifiers Viva Questions, Viva Questions on Differential Amplifiers, CMRR Viva Questions, Engineering Viva Questions, Electronics Device & Circuits,

Q.1. Define Common-mode Gain.

Ans. It may be defined as the gain parameter fur an op-amp providing the ratio of common-mode output voltage to the common-mode input voltage.

Q.2. What is Common-mode Voltage?

Ans. It is a voltage that appears the same for both magnitude and phase at two separate terminals.

Q.3. What do you mean by a Common – Mode Rejection Ratio?

Ans. It is the ratio of differential gain to common-mode gain.

Q.4. What is a Differential Amplifier?

Ans. It is an amplifier in which the output voltage is proportional to the difference between two external input voltages.

Q.5. Define Differential Input Resistance.

Ans. It is the Input resistance of op-amp between the two input terminals.

Q.6. What do you mean by Differential Gain?

Ans. It is a gain parameter for an op-amp giving the ratio of differential output voltage to the differential input voltage.

Q.7. What is the use of different amplifier stages in an op-amp?

Ans. The main purpose of the difference amplifier stage is to provide high gain to the differential mode signal and cancel the common-mode signal, i.e., it must have high CMRR.

Q.8. How do you classify ICs?

Ans. The classification of ICs is: (i) Digital IC’s (ii) Linear IC’s.

Q.9. The advent of ICs made the differential amplifier extremely popular in both BiT and MOS technologies. Why?

Ans. The main requirements of differential amplifiers are matched devices and the use of more components and IC fabrication is capable of providing matched devices and is also capable of providing a large number of transistors at a relatively low cost.

Q.10. Why are differential amplifiers preferred over single-ended amplifiers?

Ans. Basically, there are two reasons for using differential amplifiers in preference to single-ended amplifiers. First, the differential amplifiers are much less sensitive to noise and interference than single-ended circuits. The second reason for preferring differential amplifiers is that the differential configuration enables us to bias the amplifier and to couple amplifier stages together without the necessity of bypass and coupling capacitors.

Q.11. What is the basis of the classification of differential amplifier configurations?

Ans. The differential amplifier configurations are classified on the basis of the number of inputs and the way an output voltage is measured.

Q.12. What are the four differential amplifier configurations?

Ans. The four differential amplifier configurations are:

1. Dual-input, balanced-output
2. Dual-input, unbalanced-output
3. Single-input, balanced-output
4. Single-input, balanced-output

Q.13. Which one of the four configurations is not commonly used and why?

Ans. Single-input, unbalanced-output differential amplifier configuration is rarely used because of the following two reasons:

1. This configuration is identical to the CE amplifier but it requires comparatively more components and provides less voltage gain than that of CE amplifier.
2. There exists a dc output voltage without any input signal applied.

Q.14. What is the difference between balanced and unbalanced output?

Ans. When the output voltage is measured between two collectors, the configuration is referred to as a balanced output, and if it is measured across anyone collector (either C1 or C2) with respect to ground, the configuration is then said to be unbalanced output.

Q.15. Why dc analysis of a differential amplifier circuit is required?

Ans. DC analysis of a differential amplifier circuit is required for the determination of the operating point (quiescent collector current ICQ and quiescent collector-emitter voltage VCEQ).

Q.16. Why dc analysis made for dual-input, balanced-output differential amplifier configuration is also applicable for the remaining configurations too?

Ans. DC analysis for all the four configurations of the differential amplifiers is the same as long as the same biasing arrangement is used for each of them.

Q.17. What is CMRR?

Ans. CMRR is defined as the ratio of differential voltage gain to common-mode voltage gain and it is given by the expression,

CMRR=\frac{A_{id}}{A_{cm}}