The multistage amplifier is the electrical connections of two single stage amplifier single stage amplifier is one consisting of single transistor.

Vin1=Vs*(Ri1)/(Rs+Ri1)

By VDR

Vin2=Av1*Vs*[Ri1/(Ri+Rs)*Ri2/(Ri2+Ro1)]

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:

• Cascode amplifier: Cascode amplifier is the cascode connection of common emitter and common base amplifier. The transconductance of cascode amplifier is same as transconductance of of common emitter. He load of cascode amplifier is same as of common base amplifier. The cascode am[lifier has large bandwidth as compare to  common emitter amplifier hence it is called as wide band amplifier Or  high frequency amplifie.

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)]