Multistage Transistor Amplifier
A transistor circuit containing more than one stage of amplification is know as multistage transistor amplifier
In a multistage amplifier, a number of single amplifiers are connected in *cascade arrangement
i.e. output of first stage is connected to the input of the second stage through a suitable coupling
device and so on. The purpose of coupling device (e.g. a capacitor, transformer etc.) is (i) to transfer
a.c. output of one stage to the input of the next stage and (ii) to isolate the d.c. conditions of one stage
from the next stage.
Fig.1 shows the block diagram of a 3-stage amplifier. Each stage consists of one transistor and associate circuitry and is couple to the next stage through a coupling device. The name of the amplifier is usually give after the type of coupling use. e.g.
(i) In RC coupling,
A capacitor is use as the coupling device. The capacitor connects the output of one stage to the input of the next stage in order to pass the a.c. signal on while blocking the d.c. bias voltages.
(ii) In transformer coupling,
Transformer is used as the coupling device. The transformer coupling provides the same two functions (viz. to pass the signal on and blocking d.c.) but permits in addition impedance matching.
(iii) In direct coupling or d.c. coupling,
The individual amplifier stage bias conditions are so design that the two stages may be directly connected without the necessity for d.c. isolation.