Important Short Questions and Answers: Multistage Amplifiers and Differential Amplifier

MULTISTAGE AMPLIFIERS AND DIFFERENTIAL AMPLIFIER

1. Mention The Two Applications of tuned amplifiers.

i) They are used in IF amplifiers in Radio and TV receivers.

ii) They are used in wireless communication systems.

2. State two advantages and two disadvantages of tuned amplifiers.

Advantages:

i) They amplify defined frequencies

ii)  Signal to noise ratio (SNR) at output is good.

iii) They are suited for radio transmitters and receivers.

Disadvantages:

i) They are not suitable to amplify audio frequencies.

ii)  Circuit is bulky and costly.

iii) The design is complex.

3. Define CMRR.

Common mode rejection ratio (CMRR) is the ability of a differential amplifier to reject the common mode signal successfully. It is defined as the differential gain A_d and common mode gain A_O. It is denoted by letter ―ρ‖.

4. Write the expression for differential gain.

V_O= A_d(V₁-V₂)

Where A_d  = differential gain

A_d  = 20 Log₁₀(A_d ) in Db

5. What is neutralization in tuned amplifiers?

The effect of collector to base capacitance of the transistor is neutralized by introducing a signal that cancels the signal coupled through collector base capacitance. This process is called neutralization .

6. What is narrow band neutralization?

A process of cancelling the instability effect due to the collector to base capacitance of the transistor in tuned circuits by introducing a signal which cancels the signal coupled through the collector to base capacitance is called narrow band neutralization.

7. Why we go for differential amplifier? (or) What is the need of differential amplifier?

The need for differential amplifier arises in many physical measurements, in medical electronics and in direct coupled amplifier applications. In this amplifier, there will be no output voltage resulting from thermal drifts or any other changes provided, changes in both halves of the circuits are equal.

8. What are the advantages of differential amplifier?

The advantages of differential amplifier are,

·                                                     Very stable

·                                                     Low noise, low drift,

·                                                     Variations in supply voltage, temperature etc., will not change the gain of the amplitude.

·                                                     Does not require any coupling capacitor.

·                                                     Frequency response is better.

9. What are the applications of a differential amplifier?

The applications of a differential amplifier are,

·                                                                                         To measure many physical quantities,

·                                                                                         Can be used as a direct coupled amplifier,

·                                                                                         Used in operational amplifier.

10. Write the need for constant current source for difference amplifier?

The necessary for constant current source for differential amplifier to increase the common mode rejection ratio without changing the quiescent current and without lowering the forward current gain.

11. What is Common mode voltage swing?

The common mode voltage swing is defined as the maximum peak input voltage which may be applied to either the input terminal without causing abnormal operation or damage. Typically with power supplies of +6v or –6V, the common mode voltage swing should not exceed +2V.

12. Why Re is replaced by a constant current bias in a differential amplifier?

The emitter supply V_BE used for biasing purpose must become larger as Re is increased in order to maintain the quiescent current at its proper value. If the operating currents of the transistors are allowed to decrease, this will lead to higher hie values and will tend to decrease CMRR. To overcome this practical limitations RE is replaced by a constant current bias.

13. What are the advantages of Representation of Gain in Decibels.

Logarithmic scale is preferred over linear scale to represent voltage and power gains because of the following reasons :

·                 In multistage amplifiers, it permits to add individual gains of the stages to calculate overall gain.

·                 It allows us to denote, both very small as well as very large quantities of linear, scale by considerably small figures.

For example, voltage gain of 0.0000001 can be represented as -140 dB and voltage gain of 1,00,000 can be represented as 100 dB.

·                 Many times output of the amplifier is fed to loudspeakers to produce sound which is received by the human ear. It is important to note that the ear responds to the sound intensities on a proportional or logarithmic scale rather than linear scale. Thus use of dB unit is more appropriate for representation of amplifier gains.

14.            What is the coupling schemes used in multistage amplifiers?

In multistage amplifier, the output signal of preceding stage is to be coupled to the input circuit of succeeding stage. For this interstage coupling, different types of coupling elements can be employed.

These are :           

1. RC coupling

2. Transformer coupling

3. Direct coupling

15. Define i) Differential gain ii) Common mode gain

The gain with which differential amplifier amplifies the difference between two input signals is called differential gain of the differential amplifier denoted as A _D. The gain with which it amplifies the common mode signal to produce the output is called common mode gain of the differential amplifier denoted as A _C.