Differential Amplifier Interview Questions and Answers:

1. What is differential amplifier?

Ans. An amplifier, which is designed to amplify the difference between two input signals is called the differential amplifier. The differential amplifier is also referred to as a difference amplifier.

2. Advent of ICs made the differential amplifier extremely popular in both BJT and MOS technologies. Why?

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

3. 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.

4. What is the basis of 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.

5. What are differential configurations of differential amplifier?

Ans. The configurations are categorized on the basis of the number of inputs and the way an output voltage is measured. Accordingly, differential amplifier configurations may be classed as follows :

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

When both the inputs are used, the configuration is said to be dual input, otherwise single input. On the other hand, when the output voltage is measured between two collectors, it is referred to as a balanced output, because both collectors C₁ and C₂ are at the same dc potential w.r.t. ground. In case the output is measured across any one collector (either C₁ or C₂) w.r.t. ground, the configuration is known as an unbalanced output.

6. 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.

7. What is 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 any one collector (either C₁ or C₂) with respect to ground, the configuration is then said to be unbalanced output.

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

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

9. 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 same biasing arrangement is used for each of them.

10. Define CMRR.

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

11. What are the advantages of differential amplifiers with swamping resistors?

Ans. The advantages of differential amplifiers with swamping resistors are as follows :

1. The dependence of the voltage gain of the differential amplifiers on variations in dynamic emitter resistance r′_e is reduced.
2. Linearity range of the differential amplifiers is increased.

12. What are the disadvantages of just increasing R_E for improving CMRR?

Ans. It is necessary that all the resistors are carefully ratio-matched so as to have excellent CMRR. The feedback connection for inverting and noninverting amplifiers maintains the differential voltage v_d near zero volt. Increase in the value of emitter resistor R_E may cause permanent damage to the device.

13. Why R_E is replaced by a constant current source in a differential amplifier?

Ans. The resistor R_E in the differential amplifier is replaced by a constant current source in order to improve CMRR without affecting the operating point of the amplifier and without increasing the negative supply voltage -V_EE.

14. What is the current mirror?

Ans. The circuit in which the output current is forced to be equal to the input current is called current mirror circuit. Figure 2 shows block diagram and circuit diagram.

15. What is the primary advantage of using an active load?

Ans. The differential voltage gain A_d is improved and as a result, the differential gain A_d is much higher with an active load than with an ordinary resistor.

16. Explain the advantages of active load over passive load.

Ans. Advantages of active load over passive load are as follows:

1. The differential voltage gain A_d is improved and as a result the differential gain A_d is much higher with an active load than with any ordinary resistor.
2. Active load takes less area than that taken by passive load on the chip in IC fabrication and, therefore, higher packing
3. Use of active load avoids power variation, temperature variation that is caused in case of passive components.
4. For increasing input resistance so as to increase gain in case of differential amplifier or op-amp, use of active load is preferred as it provides high input resistance and so increases the gain.

17. Explain the difference between constant bias and current mirror?

Ans. The constant current bias is a circuit that provides current stabilization and therefore assures a stable operating point for the differential amplifier. The circuit in which output current is forced to be equal to be the input current is said to be a current mirror circuit. Thus, in current mirror the output current is a mirror image of input current. The current mirror is a special case of constant current bias and, therefore, can be used to provide constant current in different amplifier stages.

18. What is level translator? Draw any level translator circuit.

Ans. Level translator circuit is usually an emitter follower circuit which is used to shift the dc level at the output of the intermediate stage downward to zero volt w.r.t. ground in a cascaded differential amplifier.