The multistage amplifier is the electrical connections of two single stage amplifier single stage amplifier is one consisting of single transistor.
Voltage Gain: Av = Av1*Av2*[Ri1/(Ri+Rs)*Ri2/(Ri2+Ro1)*Rl/(+Ro2)]
Parameter of multistage amplifire:
Voltage gain: Av1 = Av1*Av2*Av3*Av4*Av5*Av6*………….*Avn
Current gain: Av1 = Ai1*Ai2*Ai3*Ai4*Ai5*Ai6*………….*Ain
Input resistance: Ri = Ri1
Output resistance: Rout = Ron
Lower cutoff frequency: FL = 1,1*√Fl1+Fl2+Fl3+.........+fln
Higher cut off frequency (fh): 1/Fh = 1.1*√ 1/Fl1+1/Fl2+1/Fl3+.........+1/fln
In multistage amplifier gain increases bandwidth decreases because of lower cutoff frequency increases and Higher cut off frequency decreases.
Special Multistage Amplifier:
DEF (darlington emitter follower):
Darlington emitter follower is the cascade connection of 2 common collector amplifier stage it has very large value of beta. The current gain of def is very large (10000). The input impedance of DEF is very large (10M ohm). The output impedance of DEF is very low (100 ohm).
𝝱def =[ 𝝱1 +𝝱2(1+𝝱1)]