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)
» JFET
MOSFET (IGFET)
n-Channel JFET
p-Channel JFET
n-Channel EMOSFET
p-Channel EMOSFET
Enhancement MOSFET
Depletion MOSFET
n-Channel DMOSFET
p-Channel DMOSFET
FET
Figure: n-Channel JFET.
The Junction Field Effect Transistor (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 N-channel) in the form of electrons.
Gate
Drain
Source
SYMBOLS
n-channel JFET
Gate
Drain
Source
n-channel JFETOffset-gate symbol
Gate
Drain
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 JFET-contd…
Small signal -Common source JFET-contd…
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
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
Small signal -Common source Amplifier -MOSFET
Analysis of CS Amplifier
A C Equivalent Circuit
Simplified A C Equivalent Circuit
Analysis of CS Amplifier-contd…
LgsmLoo
gs
ov
RvgRiv
v
vA
gain, Voltage
21 imp., Input RRRZ
Gin
dDLLmgs
ov
rRRRgv
vA ,
Dd
DdDdo Rr
RrRrZ
imp., put Out
Small Signal ‘T’ Model : NMOSFET
Small Signal Models
‘T’ Model
Analysis of CS Amplifier with Potential Divider Bias
)R||(rgAv Ddm
DR10r D,m
dRgAv
)R||(rgAv Ddm
This is a CS amplifier configuration therefore the input is on the gate and the output is on the drain. 21 R||RZi
Dd R||rZo
DdD 10RrRZo
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)
sigsigG
Ggs v
RR
Rv
sig
gs
gs
o
sig
ov v
v
v
v
v
vG
Small Signal Hybrid “π” Model : (CS)
Gin RR
Doo Rr ||R
LDogsmo RRrvgv ||||
sigG
GLDom
gs
ov RR
RRRrg
v
vG ||||
Small-signal analysis performed directly on the amplifier circuit with the MOSFET model implicitly utilized
Gin RR
sigG
GLDom
gs
o
RR
RRRrg
v
v||||
Doo Rr ||R
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.
Sm
g
Rg
vi
1
Do
Gin
RR
RR
Small-signal Analysis.
sig
i
i
gs
gs
o
sig
ov v
v
v
v
v
v
v
vG
Sm
LDm
sigG
Gv
sig
o
sigsigG
Gi
Sm
ii
Sm
mgs
LDgsmo
Rg
RRg
RR
RG
v
v
vRR
Rv
Rg
vv
Rg
gv
RRvgv
1
||
11
1
||
Voltage Gain : CS with 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
LCosigB
Bm
sig
o
Coout
Bin
RRrRrR
rRg
v
v
RrR
rRR
||||||
||
||
||
LDosigG
Gm
sig
o
Doout
Gin
RRrRR
Rg
v
v
RrR
RR
||||
||
1,
• Input Resistance is infinite (Ri=∞)
• Output Resistance = RD
• Voltage Gain is substantial
Common Source Amplifier (CS) Summary
Gin RR
sigG
GLDom
gs
o
RR
RRRrg
v
v||||
Doo Rr ||R
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
mim
i
i
iin gvg
v
i
vR
1
Dout RR
A small-signal Analusis : CG
sigm
LDm
sig
ov
sigm
sigsig
sigm
msig
sigin
ini
LDimo
sig
i
i
o
sig
ov
Rg
RRg
v
vG
Rg
vv
Rg
gv
RR
Rv
RRvgv
v
v
v
v
v
vG
1
||
11
1
||
Small signal analysis directly on circuit
The common-gate amplifier fed with a current-signal 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.
mmoout ggrR
11||
A common-drain or source-follower amplifier. : Small-signal 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
gmvsgD
1/gm
gmvsgD
Solution Small Signal Analysis : Input Resistance
Rin
Ig=0
inR
1/gm
gmvsgD
Solution Small Signal Analysis : Output ResistanceItest
ID
IG=0
IRD
Rout
test
testout I
VR
mD
m
test
D
test
testout g
R
gV
RVV
R1
||
/1
DRtest IIIC
m
testD
g
VI
1
D
testR R
VI
D
Vtest0 V
1/gm
gmvsgD
Solution Small Signal Analysis : Voltage Gain
+
- LDm
sg
o RRgv
v||+
-
vi
+
-
vsgsig
i
i
sg
sg
o
sig
o
v
v
v
v
v
v
v
v
vo
1/gm
gmvsgD
Solution Small Signal Analysis : Voltage gain
+
-
vi
+
-
vsg
LDm
m
i
sg
RRg
g
v
v
||1
1
1/gm
gmvsgD
Solution Small Signal Analysis : Voltage Gain
+
-
vi
inRsigi vv
Solution Small Signal Analysis : Voltage Gain
)R(Rg
g)||R(Rg
v
v
LDm
mLDm
sig
o
||1
1
sigi vv
LDmsg
o RRgv
v||
sig
i
i
sg
sg
o
sig
o
v
v
v
v
v
v
v
v
LDm
m
i
sg
RRg
g
v
v
||1
1
)R(Rg
)||R(R
v
v
LDm
LD
sig
o
||1
Then
1/gm
gmvsgD
Solution Small Signal Analysis : Voltage Gain
+
- LD
m
LD
i
o
RRg
RR
v
v
||1||
+
-
vi
sig
i
i
o
sig
o
v
v
v
v
v
v
sigi vv
LD
m
LD
sig
o
RRg
RR
v
v
||1||
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 current-buffering 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.