MULTISTAGE AMPLIFIERS

  When we want to achieve higher amplification than a single stage amplifier can offer, it is a common practice to cascade various stages of amplifiers, as it is shown in Fig.1.a. In such a structure the input performance of the resulted multistage amplifier is the input performance of the first amplifier while the output performance is that of the last amplifier. It is understood that combining amplifiers of various types we can create those characteristics that are necessary to fulfill the specifications of a specific application. In addition, using feedback techniques in properly chosen multistage amplifiers can further increase this freedom of the design. 

Fig.1 a) A Multistage amplifier configuration b) Small-signal equivalent of the amplifier in Fig.1a
 

According to the small signal equivalent circuit of a two stage amplifier shown in Fig.1.b, we can calculate the ac performance of the circuit.

Voltage amplification:

Current amplification:

Power amplification:

In conclusion,  the gain is the product of the gains of the individual stages (properly terminated).

 

Frequency response:

For a multistage amplifier that consists of n similar stages, the corner cut-off frequencies are given by,

where,  ω_L and ω_H are the low and high corner frequencies of the individual stages.

Fig.2  Frequency response

Noise:

·        The noise produced by the first stage of a multistage amplifier is the one that dominates the total noise figure of the amplifier.

·        Single ended noiseless amplifiers retain the same S/N ratio at the input and output of the amplifier

·        Noisy amplifiers have a worst S/N ratio at the output compared to the input