Q.What is cascaded amplifier? Why is it necessary?

When the gain of a single amplifier stage is not enough, two or more stages are connected in series so that the output of one becomes the input of the next; such a multistage arrangement is a cascaded amplifier.

[!ANSWER]

A cascaded (multistage) amplifier is two or more amplifier stages connected end-to-end; it is necessary because one stage gives limited gain, whereas cascading multiplies the individual stage gains ($A_v = A_{v1}\times A_{v2}\times\cdots$) to reach the large overall gain a weak signal needs.

[!TLDR]

A cascaded amplifier connects several amplifier stages in series to multiply their gains and obtain the large overall amplification that a single stage cannot provide.

A single amplifier stage can raise a signal only by a limited amount. In practice, signals from sources such as a microphone, a tape head or an antenna are only a few millivolts and must be built up to a level (volts) that can drive an output device. To achieve this, the output of the first stage is fed to the input of the second stage, its output to the third, and so on — the stages are said to be cascaded, forming a multistage amplifier.

Because each stage multiplies the signal, the overall voltage gain is the product of the individual stage gains:

Expressed in decibels the gains add: $G = G_1 + G_2 + G_3 + \cdots$.

Why it is necessary:

1. to obtain the very high overall gain that a single stage cannot deliver;
2. to raise a weak input signal to a usable output level;
3. to provide, along the chain, the required bandwidth and the proper input/output impedance matching (e.g. a high-input-impedance first stage and a low-output-impedance final stage). [!ANSWER] A cascaded amplifier is a chain of amplifier stages in which each stage's output drives the next; it is necessary because a single stage gives insufficient gain, while cascading multiplies the stage gains to achieve the large overall gain, level and impedance matching a weak signal requires.