Electrical Revolution

Important Questions of Transistor Amplifier

In this post, some of the important questions of transistor amplifier are given. It includes rc coupled amplifier, function of coupling capacitor, gain of multistage rc coupled amplifier, disadvantages of rc coupled amplifier, application of rc coupled amplifier, application of direct coupled amplifier, direct coupled amplifier and transistor coupled amplifier.

RC Coupled Amplifier

What is reason behind the name RC coupled amplifier?

RC coupled amplifier

The coupling between two stages in the RC coupled amplifier is done by coupling capacitor followed by shunt resistor connection so it is called as RC coupled amplifier.

Describe the function of coupling capacitor in the RC coupled amplifier.

Function of coupling capacitor

It allows AC signal to pass but blocks DC signal. To prevent DC interference between amplifier stages

Describe the main reason for low voltage and low power gain of the multistage RC coupled amplifier.

Gain of the multistage RC coupled amplifier

The total gain of the multistage RC coupled amplifier is less than the product of the gains of the individual stage or amplifiers due to fact that when effective load of the first stage is reduced because of shunting effect of the input resistance of the second stage (amplifier).
The gain of the first three stages is decreased due to shunting effect of the 4th stage or amplifier in the four-stage amplifier.
However, the gain of the 4^th stage does not affected.

Gain of the stage: 4^th stage gain > 3^rd stage gain > 2^nd stage gain > 1^st stage gain

Why the voltage gain of the RC coupled amplifier falls for lower and higher range of frequencies?

Voltage gain of the RC coupled amplifier

The voltage gain of the RC coupled amplifier is constant over the range of frequencies from 50 Hz to 20 kHz whereas its value drops for frequency less than 50 Hz and higher than 20 kHz.

At low range of frequencies, the voltage gain drops due to

The reactance of coupling capacitor becomes very high resulting only small signal will pass through one stage to the next stage.
The shunt effect is not done effectively between emitter resistance and emitter bypass capacitor due to higher reactance.

At very high range of frequencies, the voltage gain drops due to

The coupling capacitor acts as a short circuit due to very low reactance therefore it will increase the loading of the next stage resulting gain of the amplifier is reduced.
The base current increases due to reactance of the base-emitter become low at high range of frequencies.
This will reduce the current amplification factor β ( I_C = βI_B). Therefore, the voltage gain is also reduced.

What are the disadvantages of the RC coupled transistor amplifier?

Disadvantages of the RC coupled transistor amplifier

The output of the RC coupled amplifier is several hundred ohms whereas the input of the speaker is few ohms therefore impedance matching is very poor.
Low voltage and power gain due loading effect of the next stage.

State the applications of the RC coupled amplifier

Application of RC coupled amplifier

The RC coupled amplifiers are widely used as voltage amplifiers.
They are generally used in the initial stage of audio system but due to poor impedance matching property it is rarely used in the final stages of the amplification.

Direct Coupled Amplifier

State the applications of the Direct coupling amplifiers

Application of Direct coupled amplifier

The direct coupled amplifiers are used for amplifying extremely low frequencies.
They are used for amplifying photo electric current, thermo-coupled current etc.

Give reason: The capacitors and transformers are not used in the Direct coupled Amplifiers.

Direct coupled amplifiers

The direct coupled amplifiers are used particularly at very low frequency (<10Hz ).
The size of the capacitor and transformer becomes very large at low frequency therefore these types of coupling devices are not used in the direct coupled transistor amplifiers.

Why two complementary transistor (PNP and NPN ) are used in the Direct coupled amplifier?

Direct coupled amplifier

There are two complementary transistor NPN and PNP are used in the direct coupled transistor in order to make the amplifier circuit independent of the temperature changes.
The collector of the NPN transistor is connected to the base of the PNP transistor and so on...
As the temperature increases, the collector current of the NPN transistor increases resulting current amplification factor β also increases.
The above effect is compensated by PNP transistor.

Transformer Coupled Amplifier

What are the applications of the transformer coupled amplifier?

Impedance matching

The main application of the transformer coupled amplifier is impedance matching. The impedance of the output of the amplifier is very large whereas the load impedance consists of few ohms.
A step-down transformer of proper turns ratio is used in order to transfer maximum power at the last stage of the amplifier.
(The impedance of the primary of the transformer is such that it is equal to the output of the amplifier whereas the impedance of the secondary of the transformer is equal to the load resistance. )

Why the frequency response of the transformer coupled amplifier is poor?

The voltage gain of the transformer coupled amplifier is constant only for small range of frequencies. This is due to fact that the transformer primary winding is connected in series with the collected circuit therefore the output voltage is given by product of transformer primary reactance and collector current.
The output voltage and hence gain drops due to low reactance of the primary winding particularly at low frequency.
The capacitance between winding turns acts as a bypass capacitor which will reduce the output voltage and hence gain particularly at high frequency.
Therefore, the frequency response of the transformer coupled amplifier is poor.

Describe the disadvantages of the transformer coupled amplifiers.

Disadvantages of the Transformer coupled amplifier

The transformer is bulky and also expensive. Poor frequency response over wide range of frequencies.
Higher frequency distortion.
The transformer coupling produces magnetic hum or noise.