1. 4. Design of Multistage Amplifiers
4.1 Multistage RC Coupled Amplifiers
4.2 FET-BJT 2-stage RC Coupled Amplifiers
4.3 FET-BJT 3-stage RC Coupled Amplifiers

2. 4.1 Multistage RC Coupled Amplifiers
Note that in multistage amplifiers, (a) VO of the preceding stage becomes Vin of the following stage, example VO1= Vin2, (b) Rin of the following stage becomes RL of the preceding stage, example RL1= Rin2, (c) Overall gain is the product of gain of every stage.
So the design depends upon the available data between stages. For example, without knowing Rin2 of the second stage, design is not possible to meet the voltage gain of the first stage.

3. 4.2 FET-BJT 2-stage RC Coupled Amplifiers
Design Example
Draw the circuit and design the following 2-stage RC coupled amplifier.
First stage is a JFET self-bias CS amplifier circuit. Design the parameters of the JFET( IGSS, IDSS and VP,) and VDD for required RG=20MW , Av = -12 and RD = RL1 = Rin2 =2kW. Assume VD=VDD/2=10V
The above FET amplifier is RC coupled to a second stage (CE) amplifier circuit whose specifications are: b =200, VCC = 10V, optimum output voltage design, RC= RL = 2kW, RE = 0.4kW, Design bias resistances R1 and R2.
Overall performance
# What is the total voltage gain from the input of the first stage to the output of the second stage?
# Find undistorted output voltage swing at second stage output and
# The possible maximum input voltage at the first stage.

4. Draw Circuit to be designed
Self Bias CS Amp. RG=20MW , Av = -12 and RD = RL1 = Rin2 =2kW. Assume VD=VDD/2=10V
CE Amp. b =200, VCC = 10V, optimum output voltage design, RC= RL = 2kW, RE = 0.4kW

5. Because RL1 = Rin2 =2kW is available, design is possible for the first stage voltage gain and also design of RB2 of the second stage. Therefore the design can start with the first FET stage as follows;
Given; CS Amp. RG=20MW , Av = -12 and RD = RL1 = Rin2 =2kW. Assume VD=VDD/2=10V

6. Because Rin2 =2kW is available, design is possible for the RB2 of the second stage.
Given; CE Amp. b =200, VCC = 10V, optimum output voltage design, RC= RL = 2kW, RE = 0.4kW

7. Overall performance
# What is the total voltage gain from the input of the first stage to the output of the second stage?
# Find undistorted output voltage swing at second stage output and
# The possible maximum input voltage at the first stage.
10V 2k
0.4k
73k
19k
4.24mV(pp)
71W
20MW
5.88V(pp)
Av1=-12
Av2=-113
AvT=1357

8. 4.3 FET-BJT-FET 3-stage- RC Coupled Amplifiers
To design an amplifier having (a) very high Rin & (b) very high voltage gain & (c) very large signal output voltage swing driving a very low load resistance, following stages are selected.
SF Amp. To provide high Rin3 and Voltage divider bias to provide large output voltage swing to drive RS3= RL = 1000W
Self bias CS Amp. To provide very high Rin but small signal Vo since it is a preamplifier stage Take AV = -12
CE Amp. For high Av
Take RC=2kW Rin3 = 20kW
For maximum voltage transfer from 2nd stage to 3rd stage

9. Draw and Design RG , RS , RD = RL and VDD of the first stage JFET self-bias CS amplifier circuit. FET data is given as IGSS=300nA, IDSS=5mA and VP=-3V, The required specification is VD=VDD/2 and Av = -12 RG
RD = RL RS
VDD

10. Draw and design second stage (CE) amplifier circuit
Given specifications are: b = 200, VCC = 10V, RC= 2 k W, RL = 20 k W , RE = 0.4kW, Rin = 10.3k W. Design bias resistances R1 and R2,
# find voltage gain,and undistorted output voltage swing of this stage. RG
Vo2 (pp)
R12
Av2
R22

11. Draw and design R13 , R23 , and VDD3 of the third stage JFET voltage-divider bias SF amplifier circuit if the FET data is IGSS=1mA, IDSS=10mA and VP=-3V, The required specification is VS=VDD/2 to drive a load of RS = RL 1kW
# Find Ro of your amplifier and # maximum input voltage at the first stage.
Check Rin3 < 300k with IGSS=1mA
Ro < 1k so it can drive RL=1kW load easily
Vin1 (pp)
R13
AvT
R23
VDD3

12. Overall performance
# Find AvT of your amplifier and # maximum input voltage at the first stage and # check Ro whether it can drive RL=1kW load easily.
Ro < 1k so it can drive RL=1kW load easily
Vin1 (pp)
AvT
1MW
10.13kW
360W
3.33kW
16.49kW
2kW
0.4kW
10V
50.65V
48.8kW
33.9kW
1kW
3 mV 5V
Av1=-12
Av2=-164
Av3=0.825
AvT=1623.6