Test: Multistage Amplifier - Electronics and Communication Engineering (ECE) MCQ

The transformer coupling has a poor frequency response. The gain varies considerably with frequency. The gain is constant only over small range of frequencies. Thus transformer coupling introduces frequency distortion; due to which its frequency response is poor.

RC coupled amplifiers have low voltage and power gain. It is because the low resistance presented by the input of each stage to the preceding stage decreases the effective load resistance and hence the gain. Thus its efficiency is reduced.

It uses the resistor and capacitor which are not expensive so the cost is low. But it has poor impedance matching because its output impedance is several times larger than the device; at its terminal end. It is unsuitable for low frequency application.
 

The overall gain of a multistage amplifier is given as the product of the gain of the individual stages.
Gain (A) = A₁ x  A₂  x A₃ …… x  A_n
Alternately, if the gain of each stage is given in db
The overall gain of the amplifier is the sum of gain of each stage
Gain in db = A₁ + A₂ + A₃ (db)
= 10 + 6 + 15 = 31db.

Usually the impedance of an output device is a few ohms whereas output impedance of the transistor is several 100 ohms. In order to match the impedance a step down transformer of proper turn’s ratio is used. The impedance of secondary of the transformer is made equal to the load impedance and primary impedance equal to the output impedance of the transistor.

The output of first amplifier stage is the input to next stage. In this way overall voltage gain can be increased, when number of amplifier stage is used in succession, it is called a multistage amplifier. The load of first amplifier is the input resistance of the second amplifier. Thus overall gain is reduced.

In RC or transformer coupling, a capacitor / transformer is used as coupling device which connects output of first stage with input of second stage. Its function is to pass the a.c signal and blocks d.c. bias voltage.

A multistage amplifier circuit affects the high input impedance of a common source stage combined with the input to output isolation of a common gate stage. A radio receiver has more than one stage of amplification because it is required to restore the characteristics of a radio signal over various channels.

Db scale is logarithmic. Voltage gain increases exponentially with frequency so using a linear scale means that we need to work with large values of gain, corresponding to small values of frequency.

X_c = the electrical size of coupling capacitor
Relation of X_c with the signal frequency (f):
X_c = 1 \X_c is inversely proportional to f.
At low frequencies, Xc becomes very large; output reactance of capacitor increases.
The voltage across load resistance also reduces because some voltage drop takes place across X_c.
Thus output voltage reduces. Therefore gain is very low.