Fet multistage amplifiers, Lecture notes of Basic Electronics

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Multistage Amplifiers

(FET)

Multistage Amplifiers

• In most situations, a single-transistor amplifier

cannot meet all given specifications.

• The required voltage gain often exceeds the

amplification factor of a signal transistor, or the

combination of voltage gain, input resistance and

output resistance cannot be met simultaneously.

• To achieve the desired specification/performance,

several stages of amplification may be required.

• Multiple transistor stages are connected in such a

way that the output of one stage is the input to the

next stage and so on.

Cascade Configuration

• Transistor amplifier circuits are connected in series , or cascaded.
• This may be done either to increase the overall small-signal voltage gain or to provide an overall voltage gain greater than 1, with a very low output resistance.

5 A three-stage capacitor-coupled cascaded amplifier

A three-stage capacitor-coupled cascaded amplifier

CS CS CD

DC biasing circuit

Small-Signal Equivalent Circuit

gm1vgs gm2vgs R 1 ||R 2 RD||RL Rin R out RL

Exercise 1

The transistor parameters in the following circuit are:

2 2 2

K n 1  mA V Kp  mA V VTN  V VTP  Vand   

The circuit parameters are: VDD  10 V , RS 1  4 k  andRin  200 k 

I (^) DQ 1  0. 4 mA , IDQ 2  2 mA , VDSQ 1  4 VandVSDQ 2  5 V v o i A  v / v (a) Design the circuit such that (b) Calculate the small-signal voltage gain