1. ANALOGUE ELECTRONICS I
EMT 112/4
Frequency Response of FET Amplifiers

2. LOW-FREQUENCY AMPLIFIER RESPONSE
Input RC Circuit
Output RC Circuit
Bypass RC Circuit

3. FET Amplifier +VDD RD C2 Vo RSi C1 RL RG Vi C3 RS
Common-source FET amplifier

4. Low-frequency Equivalent Circuit
Input RC circuit
Output RC circuit
Bypass RC circuit

5. The cutoff frequencies defined by the input , output and bypass circuits can be obtained by the following formulas:
Input RC circuit
where
RC1=RSi+RG
Input RC circuit

6. Output RC circuit
where
RC2=RD+RL
Output RC circuit

7. Bypass RC circuit
where
RC3=RS||1/gm
Bypass RC circuit

8. Low cut-off frequency Hence,
fC = the largest of the three low cut-off frequency

9. Example
Determine the lower cutoff frequency for the FET amplifier. Given K = 0.4mA/V2, VTN= 1V,  = 0 RL RS RD RG C1 C2 Vi
+VDD C3
RSi Vo
1K
20V
10K
4.7K
2.2K
1M
0.5F
0.01F
2F

10. Solution
Input RC circuit
Bypass RC circuit
Output RC circuit

11. Since fc in bypass RC circuit is the largest of the three cutoff frequencies, it defines the low cutoff frequency for the amplifier:
 fc = Hz

12. HIGH-FREQUENCY AMPLIFIER RESPONSE
Input RC Circuit
Output RC Circuit

13. hi-frequency hybrid- model
HIGH-FREQUENCY
Small capacitances exist between the gate and drain and between the gate and source. These affect the frequency characteristics of the circuit. ro
Cgd
Vgs
gmVgs
Cgs - +
hi-frequency hybrid- model

14. Basic data sheet for the BS 107 n-MOSFET
Cgs = Ciss - Crss
Cgd = Crss

15. Unity-Gain Bandwidth
Unity gain frequency / bandwidth, fT is defined as a frequency at which the magnitude of the short-circuit current gain goes to 1
It is a parameter of FET & is independent of circuit

16. FET Amplifier
In high-frequency analysis, coupling and bypass capacitors are assumed to have negligible reactances and are considered to be shorts. vo RL RS RD R1 C1 C2 vi
+VDD C3
RSi R2

17. High-frequency hybrid- model with Miller effect
RTH1
RTH2
RSi Vo
R1||R2 Vi
Cgs
CMi
CMo
RD||RL
gmVgs
A : midband gain

18. The cutoff frequencies defined by the input and output circuits can be obtained by first finding the Thevenin equivalent circuits for each section as shown below:
RTH2
RTH1
Cin
Cout vi vi
(a) Input circuit
(b) Output circuit
where RTH1 = RSi||R1||R2 and
Cin = Cgs + CMi
where RTH2 = RD||RL and
Cout = CMo

19. Example
Find the cutoff frequency of the input and output RC circuit for the FET amplifier in figure below. Given that Cgd=0.1pF, Cgs=1pF, K =0.5mA/V2 and VTN=2V, =0. vo RL RS RD R1 C1 C2 vi
+VDD C3
RSi R2
4 k
234 k
10 k
166 k
0.5 k
20 k
10 V

20. Solution
DC Analysis

21. Input RC circuit Midband gain
 Thevenin’s equivalent resistance at the input
 total input capacitance
 upper cutoff frequency introduced by input capacitance

22. Output RC circuit  Thevenin’s equivalent resistance at the output
 total output capacitance
 upper cutoff frequency introduced by output capacitance