Week 6 – Chapter 3
FET Small-Signal
Analysis
Mohd Shawal Jadin
FKEE UMP © 2009
FET Small-Signal Model
Transconductance
The relationship of VGS (input) to ID (output) is called transconductance.
The transconductance is denoted gm.
Transfer
Curve
gm 
ID
VGS
Graphical Determination of gm
Mathematical Definition of gm
ID
gm 
ΔVGS
Using differential calculus
I D  K (VGS  VTR )2
gm  2K(VGS  VTR )
FET Impedance
Input Impedance Zi:
• Very large to assume input terminal
approximate an open circuit
Output Impedance Zo:
yos: admittance equivalent circuit
parameter listed on FET
specification sheets.
rd 
VDS
ID VGS  constant
Zi   Ω
Zo  rd 
1
yos
FET Specification
FET AC Equivalent Circuit
JFET Fixed-Bias Configuration
The input is on the gate and the output is on the drain.
JFET Fixed-Bias Configuration
Once again: same step as BJT to redraw the network to AC equivalent circuit.
Capacitor – short circuit
DC batteries VGG and VDD are set to zero volts by a short-circuit
equivalent
AC Equivalent Circuit
AC Equivalent Circuit
Impedances
Input Impedance:
Zi  RG
Output Impedance:
Zo RD || rd
Zo  RD
rd 10RD
Voltage Gain
Vo
Av 
 gm(r d ||RD)
Vi
Vo
Av 
 gmRD
Vi
rd 10R D
Phase Relationship
A CS amplifier configuration has a 180-degree phase shift between input and output.
Example
Fixed-bias configuration has an operating point defined
by VGSQ = -2V and IDQ = 5.625 mA, with IDSS = 10mA and VP
= -8V. The value of yos is provided as 40 µS. Determine:
a) gm
b) Zi
c) Zo
d) AV
e) AV ignoring the effects
of rd
Solution
JFET CS Self-Bias Configuration
This is a CS amplifier configuration therefore the input is on the gate and the output is on
the drain.
AC Equivalent Circuit
Impedances
Input Impedance:
Output Impedance:
Zi  RG
Zo rd || RD
Zo  RD
rd 10RD
Voltage Gain
Av  gm(rd || RD)
Av  gmRD
rd 10RD
Phase Relationship
A CS amplifier configuration has a 180-degree phase shift between input and output.
JFET CS Self-Bias Configuration – Unbypassed Rs
If Cs is removed, it affects the gain of the circuit.
AC Equivalent Circuit
Impedances
Input Impedance:
Output Impedance:
Impedances
Voltage Gain
Voltage Gain
Example
Solution
Solution
JFET CS Voltage-Divider Configuration
This is a CS amplifier configuration therefore the input is on the gate and the output is on
the drain.
AC Equivalent Circuit
Impedances
Input Impedance:
Zi R1 || R2
Output Impedance:
Zo rd || RD
Zo  RD
rd 10RD
Voltage Gain
Av  gm(rd || RD)
Av  gmRD
rd 10RD
JFET Source Follower (Common-Drain) Configuration
In a CD amplifier configuration the input is on the gate, but the output is from the source.
AC Equivalent Circuit
Impedances
Input Impedance:
Zi  RG
Output Impedance:
1
Zo rd || RS ||
gm
Zo  RS ||
1
gm
rd 10RS
Voltage Gain
Vo
gm(rd || RS)
Av 

Vi 1 gm(rd || RS)
Vo
gmRS
Av 

Vi 1 gmRS
rd 10RS
Phase Relationship
A CD amplifier configuration has no phase shift between input and output.
JFET Common-Gate Configuration
The input is on source and the output is on the drain.
AC Equivalent Circuit
Impedances
Applying Kirchhoff’s voltage law around the output perimeter and Kirchhoff’s current
law at node a ::
Impedances
Input Impedance:
 rd  RD 
Zi  RS || 

1 gmrd 
Zi  RS || ( 1 )
gm
rd 10RD
Output Impedance:
Zo RD || rd
Zo  RD
rd 10RD
Voltage Gain
Applying Kirchhoff’s current law at node b ::
RD 

g
mRD 
Vo 
rd 
Av 

Vi
 RD 
1 rd 
Av  gmRD
rd 10RD
Phase Relationship
A CG amplifier configuration has no phase shift between input and output.
Depletion-Type MOSFETs
1. D-MOSFETs have similar AC equivalent models.
2. The only difference is that VSGQ can be positive for nchannel devices and negative for p-channel devices.
3. This means that gm can be greater than gm0.
D-MOSFET AC Equivalent Model
• Find
–
–
–
–
Example
VGSQ and IDQ
Determine gm and compare to gm0
rd
Find Zi, Zo, Av
Enhancement-Type MOSFETs
There are two types of E-MOSFETs:
nMOS or n-channel MOSFETs
pMOS or p-channel MOSFETs
E-MOSFET AC Equivalent Model
Forward transfer admittance
gm and rd can be found in the specification sheet for the FET.
E-MOSFET CS Drain-Feedback Configuration
AC Equivalent Circuit
Impedances
Input Impedance:
Zi 
RF
1 gmRD
RF  rd || RD
Zi 
1 gm(rd || RD)
Output Impedance:
Zo RF || rd || RD
RF  rd || RD, rd 10RD
Zo  RD
RF  rd || RD, rd 10RD
The calculation
From Kirchoff's Current Law at point D,
Vo
Ii  gmVGS 
VGS  Vin
rd RD
and
Ii  Vi Vo
RF
We ' ll find that,
Ii 


Vi  rd R D I i  gmVi 
RF
rearranging ,
Vi 1  gmrd R D   I i RF  rd R D 
so,
Zi 
RF
1  gmRD
RF  rd || RD, rd  10RD
Zo  RF||rd||RD
Zo  RD
RF  rd||RD, rd  10RD
F romKirchoff's CurrentLaw at point D,
Vo
Ii  gmVGS 
VGS  Vin
rd RD
and
Ii  Vi Vo
RF
We' ll find that,
Vi Vo  gmVGS  Vo
RF
rd RD
rearranging ,
 1
1

1
Vo 


V

g
m

i

 rd RD RF 
 RF

so ,
1

1

 R  gm
 R  gm


Vo
F
F




AV 


  gm rd RD R

Vi
 1


F
1

1

 rd RD R 


F 

 rd RD R 
F


  gmRD RF   rd||RD, rd  10RD





The AC analysis of E-MOSFET
The relationship between output currentand
controlling voltage is as below;
ID  k VGS  VGS  Th 
2
We know that gm is definedby the slope of drain
characteristics,
gm 
ID
VGS
The derivationof above equation will determine gm
at the operatingpoint;
gm 

ID
d
2

k VGS  VGS Th 
VGS dVGS

 2k VGSQ  VGS Th 
Remember that, the biasing arrangement are limited for E-MOSFET
Voltage Gain
Av  gm(RF || rd || RD)
Av  gmRD
RF  rd || RD, rd 10RD
Phase Relationship
This is a CS amplifier configuration therefore it has 180-degree phase shift between input
and output.
Do it
•Determine input and output and also AV
impedance for k=0.3X10-3
+VDD
(+16V)
2.2kΩ
Vo
10MΩ
VGS(Th)=3V
rd=100kΩ
Vi
Zi
E-MOSFET CS Voltage-Divider Configuration
AC Equivalent Circuit
Impedances
Input Impedance:
Output Impedance:
Zi R1 || R2
Zo rd || RD
Zo  RD
rd 10RD
Voltage Gain
Av  gm(rd || RD)
Av  gmRD
rd 10RD
Phase Relationship
This is a CS amplifier configuration therefore it has 180-degree phase shift
between input and output.
Solution
E-MOSFET CS Voltage-Divider Configuration
AC Equivalent Circuit
Impedances
Input Impedance:
9.52]
Output Impedance:
9.53]
Zi R1 || R2
[Formula
Zo rd || RD
Zo  RD
rd 10RD
[Formula
Voltage Gain
Av  gm(rd || RD)
Av  gmRD
rd 10RD
[Formula 9.55]
[Formula 9.56]
Summary Table
Summary Table
Try yourself
•Design a self-bias network that have
gain of 10. The device should be biased
at VGSQ=1/3VP
+VDD
(+20V)
RD
Vo
IDSS =12mA
VP=-3V
rd=40kΩ
Vi
10MΩ
Rs
Solution