A transistor circuit having more than one stage of amplification is known as a multistage transistor amplifier. The number of single amplifiers is linked to the cascade arrangement. That is the output of the 1st stage is connected to the input of the 2nd stage through a suitable coupling device and so on.
| Name of coupling | Name of multistage amplifier |
| RC coupling | RC coupled amplifier |
| Transformer coupling | Transformer coupled amplifier |
| Direct coupling | Direct couple amplifier |
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Gain of multistage amplifier
The ratio of the output power quantity to the input one of the amplifier is know as its gain.
Therefore the gain is the multistage transistor amplifier is equal to the product of gain of individual stages.
If G1 , G2 and G3 are the individual voltage gains of a three-stage amplifier then the total voltage gain is G = G1 × G2 × G3.
What is frequency response of an amplifier?
The curve between voltage gain and signal frequency all the amplifier is known as frequency response.
The gain of the amplifier increases as the frequency increases from zero to a maximum at ‘fr’ which is know as the resonant frequency. If the frequency of the signal increases beyond ‘fr’ the gain decreases.
Decibel gain of an amplifier
The common logarithm ( log10 ) of power gain is know as decibel or bel power gain.
Advantages of decibel
The unit dB ( decibel ) is a logarithmic unit. The response of our ear is also logarithmic that is the loudness of the sound heard by the ear is not proportional to the intensity of the sound.
But according to the logarithm of the intensity of the sound. When the gains are express in dB, the overall gain of a multi-stage amplifier is the gains of the personal phase in dB .
| So Total gain (dB) is = 1st stage gain in dB + 2nd stage gain in dB |
Bandwidth of multistage amplifier
The range of frequency at which voltage gain is equal to or greater than 70.7% or maximum gain is known as bandwidth.
From the figure F1 – F2 is the bandwidth. So F1 is know as lower cutoff frequency and F2 is know as upper cutoff frequency. The drop in voltage gain from maximum gain is
= 20 log10 100 − 20 log10 70.7
= 20 log10 100 / 70.7 dB
and = 20 log10 1.4142 dB
= 3 dB
Therefore bandwidth is the range of an amplifier’s frequency over which its voltage gain drops by 3dB from maximum gain.
F1 or F2 is also know as 3 dB frequency or half power frequency. So audio frequency is = 20 Hz to 20 kHz , and gain = output/ input.
RC coupled transistor amplifier
In the figure a coupling capacitor ( CC ) is use to connect the output of 1st stage to the base of the 2nd stage and so on.
Since the coupling from one phase to another is achieve by a coupling capacitor followed by a connection to a shunt resistor. Hence it is called RC coupled amplifier.
Resistance R1, R2 and RE from the biasing and stabilization network. Hence the emitter bypass capacitor provides a low reactance path to the signal.
Operation :
When a. c. signal is applied to the base of gain to the base of the 1st transistor it appears in the amplifying from across its collector load RC.
The amplified signal develop across the RC is pass to the base of the next stage, to the coupling capacitor. So the 2nd stage does further amplification for the signal.
But here the total gain is less than the product of the gains of the individual steps. Because when the second phase is made to follow the first phase the effective load resistance of the first phase is reduced.
Due to the shunting effect of the input resistance 2nd phase.
What is frequency response of RC coupled amplifier?
From the figure it is clear that voltage gain drops a low frequency (less than 50 Hz) and high frequency ( greater than 20 kHz). While it is uniform over the mid-frequency range (50 Hz to 20 kHz).
At low frequency ( less than 50 Hz ) the reactance of coupling capacitor is quit high. So the smaller part of the signal will go from one phase to the other.
The CE cannot shunt the emitter resistor effectively due to its large reactance at low frequency. At high frequency above 20 kHz the reactance of coupling capacitor is very low and it behaves as a short circuit.
This increases the loading effect of the next stage and reduces the voltage gain. At mid-frequency (50 Hz to 20 kHz) the effect of the coupling capacitor is such that a uniform voltage gain is maintain.
Therefore in this range as the frequency increases reactance of CC decreases which increases the gain. However at the same time more loading of the 1st stage and hence lower gain.
So this two factors almost cancel each other out resulting in a uniform gain at mid frequency.
Advantages
- It has excellent frequency response, stable voltage gain over the audio frequency range which is most important for speech, music etc.
- It has lower cost.
- The circuit is very compact that is size is small.
Disadvantages
- It has lower voltage and power gain ( because effective load ).
- They have a tendency to make noise with age, especially in moist climax.
- Impedance matching is poor.
Application
It is widely use as voltage amplifiers that is initial stage of public address system.
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Transformer coupled transistor amplifier
Transformer couplings are generally use when the load is small and are mostly use for power amplification. In the figure a coupling transformer is use to feed the output of one stage into the input of the next stage.
Hence the primary of this transformer is made the collector load and the secondary gives the input to the next stage.
Operation :
When a A. C. the signal is first applied to the base of the transistor, it appears as amplified in the primary of the coupling transformer.
Hence the voltage develop in the primary is transferred by the secondary to the input of the next stage. So the second stage performs the amplification again in the same manner.
Transformer coupled amplifier frequency response
From the figure it is clear that the frequency response is poor that is the voltage gain is constant only over a small range of frequency. So at low frequency the reactance of the primary starts decreasing resulting in reduction of gain.
The high frequency, the capacitance between the turns of the winding act as a bypass condenser to reduce the output voltage and hence gain.
Advantages
- No signal power is lost across the collector or base resistor.
- An excellent impedance matching can be achieve.
- Due to excellent impedance matching it provides higher gain.
Disadvantages
- It has a poor frequency response ( That is gain varies with frequency ).
- The coupling transformers are heavy and expensive at audio frequencies.
- Frequency distortion is high that is low frequency signals are less amplified than high frequency signals.
Application
- Transformer couplings are mostly use for impedance matching.
- In general the last stage of a multistage amplifier is the power stage.
NOTE :-
So here it is made to transfer maximum power to the output device that is a loudspeaker. For maximum power transfer the impedance of power source should be equal to that of load.
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Comparison :
| S. No. | Particulars | RC coupling | Transformer coupling |
| 1 | Frequency response | Excellent in audio frequency range | Poor |
| 2 | Cost | Less | More |
| 3 | Space and weight | Less | More |
| 4 | Impedance matching | Poor | Excellent |
| 5 | Use or application | For voltage amplification | For power amplification |
So in this post we read about Multistage Transistor Amplifier, In this type of amplifier is connect in a cascade arrangement and has multiple stages in which the output of the first stage is connect to the input of the second stage each stage contains a transistor. The name of the multistage transistor amplifier is usually given after the type of coupling , So this was about multistage transistor amplifier, RC coupled and transformer coupled amplifier. Thank you.