UNIT III JFET and MOSFET Amplifiers OUTLINE Small signal Analysis of JFET amplifiers Small signal...
Transcript of UNIT III JFET and MOSFET Amplifiers OUTLINE Small signal Analysis of JFET amplifiers Small signal...
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.