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UNIT III
JFET and MOSFET Amplifiers
OUTLINE
• Small signal Analysis of JFET amplifiers
• Small signal Analysis of MOSFET and JFET
• BICMOS cascode Amplifier
FET ( Field Effect Transistor)
•
•
Unipolar device i. e. operation depends on only
one type of charge carriers.
Voltage controlled Device (gate voltage controls
drain current).
•
Very high input impedance (109-1012 ).
•
Source and drain are interchangeable in most
Low-frequency applications.
•
Low-power consumption.
•
•
Less Noisy as Compared to BJT.
Very small in size, occupies very small space in
Ics.
Types of Field Effect Transistors
(The Classification)
»
FET
JFET
MOSFET (IGFET)
Enhancement
MOSFET
n-Channel
EMOSFET
p-Channel
EMOSFET
n-Channel JFET
p-Channel JFET
Depletion
MOSFET
n-Channel
DMOSFET
p-Channel
DMOSFET
The Junction Field Effect Transistor (JFET)
Figure: n-Channel JFET.
JFET
• There are two basic configurations of junction
field effect transistor, the N-channel JFET and the
P-channel JFET.
• The N-channel JFET’s channel is doped with
donor impurities meaning that the flow of current
through the channel is negative (hence the term Nchannel) in the form of electrons.
SYMBOLS
Gate
Gate
Gate
Source
n-channel JFET
Drain
Drain
Drain
Source
n-channel JFET
Offset-gate symbol
Source
p-channel JFET
Small signal Analysis
• The coupling capacitors bypass capacitor are
short circuitED
•
Short the DC supply voltage
•
Replace the FET with the hybrid-p model
Common source JFET
Small signal -Common source JFETcontd…
Small signal -Common source JFETcontd…
Source Follower JFET
Small signal equivalent-Source Follower
JFET
Av= gmRs/(1+gmRs)
Common Gate JFET
Small signal model-Common Gate JFET
AV= -gmRd
Rin=1/gm
Self bias of JFET
Small signal equivalent -Self bias of
JFET-contd…
Enhancement MOSFET showing channel length L and
channel width W.
MOSFET- SYMBOL
Small-signal equivalent circuit for FETs.
FET small-signal equivalent circuit that accounts for the
dependence of iD on vDS.
MOSFET Characteristics
Common source Amplifier -MOSFET
Small signal -Common source Amplifier
-MOSFET
For drawing an a c
equivalent circuit of
Amp.
•Assume all Capacitors
C1, C2, Cs as short
circuit elements for ac
signal
•Short circuit the d c
supply
•Replace the FET by
its small signal model
Analysis of CS Amplifier
A C Equivalent Circuit
Simplified A C Equivalent Circuit
Analysis of CS Amplifier-contd…
v
Voltage gain, A  o
v v
gs
Input imp., Z
v  i R  g v R
o
A 
v
o L
v
o
v
1
G
2
m gs L
 g R , R  R r
m L
gs
in
R R R
L
D d
Out put imp., Z  r R 
o
d
D
r R
d D
r R
d
D
Small Signal ‘T’ Model : NMOSFET
Small Signal Models
‘T’ Model
Analysis of CS Amplifier with Potential Divider Bias
Av  gm(rd || RD)
This is a CS amplifier configuration therefore the
input is on the gate and the output is on the drain.
Zo  rd || RD
Av  gm(rd || RD)
Av  gmRD,  r  10 R
d
Zi  R1 || R2
Zo  RD
D
rd 10RD
An Amplifier Circuit using MOSFET(CS Amp.)
A small signal equivalent circuit of CS Amp.
Common Source Amplifier (CS)
• Signal ground or an ac earth is at the source through a
bypass capacitor
• Not to disturb dc bias current & voltages coupling
capacitors are used to pass the signal voltages to the
input terminal of the amplifier or to the Load
Resistance
• CS circuit is unilateral
– Rin does not depend on RL and vice versa
Small Signal Hybrid “π” Model : (CS)
vo vo vgs
Gv   
vsig vgs vsig
RG
vgs 
vsig
RG  Rsig
Small Signal Hybrid “π” Model : (CS)
Rin  RG
R o  ro || RD
vo   g m v gs ro || RD || RL 
 RG 
vo

Gv    g m ro || RD || RL 

vgs
R

R
 G sig 
Small-signal analysis performed directly on the amplifier
circuit with the MOSFET model implicitly utilized
Rin  RG
R o  ro || RD
 RG 
vo

  g m ro || RD || RL 


v gs
 RG  Rsig 
Common-source amplifier
with a resistance RS in the source lead
The Common Source Amplifier with a
Source Resistance
• The ‘T’ Model is preferred, whenever a resistance is
connected to the source terminal.
• ro (output resistance due to Early Effect) is not
included, as it would make the amplifier non
unilateral
Small-signal equivalent circuit with ro neglected.
i
vg
1
 RS
gm
Small-signal Analysis.
Rin  RG
Ro  RD
Voltage Gain : CS with RS
vo
vo vgs vi
Gv 



vsig vgs vi vsig
vo   g m v gs RD || RL 
1
gm
vi
v gs 
vi 
1
1  g m RS
 RS
gm
RG
vi 
vsig
RG  Rsig
 RG  g m RD || RL  
vo


 Gv  



vsig
 RG  Rsig  1  g m RS 
Common Source Configuration with Rs
• Rs causes a negative feedback thus improving
the stability of drain current of the circuit but
at the cost of voltage gain
• Rs reduces id by the factor
– (1+gmRs) = Amount of feedback
• Rs is called Source degeneration resistance as
it reduces the gain
Small-signal equivalent circuit directly on
Circuit
BJT / MOSFET
Rin  RB || r
  ,   1
Rin  RG
Rout  ro || RC
Rout  ro || RD
vo
RB || r
ro || RC || RL 
 gm
vsig
RB || r  Rsig
vo
RG
ro || RD || RL 
 gm
vsig
RG  Rsig
Common Source Amplifier (CS) Summary
• Input Resistance is infinite (Ri=∞)
• Output Resistance = RD
• Voltage Gain is substantial
Rin  RG
R o  ro || RD
 RG 
vo

  g m ro || RD || RL 
R R 
v gs
sig 
 G
MOSFET -Source follower
.
Small-signal ac equivalent circuit for the
source follower
.
Equivalent circuit used to find the output
resistance of the source follower.
Common-gate amplifier.
Small signal-Common Gate
Small-signal ‘π’ models for the MOSFET
Voltage swing limitation
• Up swing limited by transistor going in to cut off
• Vout (max)= VDD
• Lower swing limited by MOSFET entering in to
linear region
• Vou(min)-VGG-VT
A common-gate amplifier based on the circuit
Common Gate (CG) Amplifier
• The input signal is applied to the source
• Output is taken from the drain
• The gate is formed as a common input & output port.
• ‘T’ Model is more Convenient
• ro is neglected
A small-signal equivalent circuit
A small-signal Analusis : CG
vi
vi
1
Rin  

ii g m vi g m
Rout  RD
A small-signal Analusis : CG
vo vo vi
Gv 
 
vsig vi vsig
vo  g m vi RD || RL 
1
gm
vsig
Rin
vi 
vsig 
vsig 
1
Rin  Rsig
1

g
R
m
sig
 Rsig
gm
vo g m RD || RL 
Gv 

vsig 1  g m Rsig
Small signal analysis directly on circuit
The common-gate amplifier fed with a currentsignal input.
Summary-CG
• CG has much higher output Resistance
• CG is unity current Gain amplifier or a Current
Buffer
• CG has superior High Frequency Response.
.
A common-drain or source-follower
amplifier
Small-signal equivalent-circuit model
Small-signal Analysis : CD
A common-drain or source-follower amplifier
:output resistance Rout of the source follower.
Rout
 1  1
 ro ||   
 gm  gm
A common-drain or source-follower amplifier. : Smallsignal analysis performed directly on the circuit.
Common Source Circuit (CS)
Common Source Circuit (CS) With RS
Common Gate Circuit (CG)
Current Follower
Small Signal Model MOSFET : CD
Small Signal Analysis CD
1/gm
gmvsg
D
Solution Small Signal Analysis : Input
Resistance
1/gm
Ig=0
gmvsg
Rin
Rin  
D
Solution Small Signal Analysis : Output Resistance
Itest
1/gm
ID
IG=
0
I
R
gmvsgD
0V
D
Vtest
Rout
Rout
Vtest

I test
I test  I RC  I D
I RD
Vtest

RD
Vtest
ID 
1
gm
Rout
Vtest
1

 RD ||
Vtest Vtest
gm

RD 1 / g m
Solution Small Signal Analysis : Voltage
Gain
+
vsg
-
1/gm
-
+
vi
-
vo
vo vsg vi



vsig vsg vi vsig
D
gmvsg
vo
+
vo
  g m RD || RL 
vsg
Solution Small Signal Analysis : Voltage
gain
+
vsg
1/gm
+
vi
gmvsg
D
-
vsg
vi
1

1
gm
gm
 RD || RL 
Solution Small Signal Analysis : Voltage Gain
1/gm
+
vi
gmvsg
D
-
vi  vsig
Rin  
Solution Small Signal Analysis : Voltage
Gain
vo
vo vsg vi



vsig vsg vi vsig
vsg
vi
1

1
gm
gm
 RD || RL 
vo
  g m RD || RL 
vsg
vo
  g m(RD||R L ) 
1
vsig
vi  vsig
Then
 1
gm
gm
 (RD || RL )
vo
(RD||R L )

1  (RD || RL )
vsig
gm
Solution Small Signal Analysis : Voltage
Gain
1/gm
+
vi
gmvsg
+
-
vo vo vi
 
vsig vi vsig
D

vo
RD || RL 

1  RD || RL 
vi
gm
vi  vsig

vo
RD || RL 

1  RD || RL 
vsig
gm
BICMOS
BICMOS -contd…
• Combining the high gain of BJT and infinite
impedance of MOSFET will lead to BiCMOS
differential amplifier design.
• Rs = typical 100 KW
• BICMOS cascode amplifier has overall voltage
gain of C-S, but with frequency response
comparable to CB Amplifier.
BICMOS -contd…
• The basic idea is to combine the high Rin and
large transconductance (g m) of a commonsource
(common-emitter) amplifier with the currentbuffering property and superior high-frequency
response of the common-gate (common-base)
circuit
BICMOS -contd…
• In response to input signal voltage vi, the CS
transistor Q1 conducts a current signal gm1 vi in
its drain terminal and feeds it to the source of the
CG transistor Q 2 (cascode transistor).
• Q 2 passes signal to its drain and to the load RL. Q
2 acts as a buffer, presenting low Rin to the drain
of Q1 and providing high Rout at output.