Small Signal Model MOS Field-Effect Transistors (MOSFETs)
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Transcript of Small Signal Model MOS Field-Effect Transistors (MOSFETs)
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Small Signal ModelMOS Field-Effect Transistors (MOSFETs)
Quiz No 3 DE 27 (CE)
(a) Draw small signal model (4)(b) Find expression for Rout (2) (c) Prove vo/vsig = (β1α2RC)/(Rsig+rπ) (4).
Rout.
20-03-07
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Enhancement-type NMOS transistor:
Figure 4.2 The enhancement-type NMOS transistor with a positive voltage applied to the gate. An n channel is induced at the top of the substrate beneath the gate.
Enhancement-type NMOS transistor:
MOSFET Analysis
iD = iS, iG = 0
Large-signal equivalent-circuit model of an n-channel MOSFET : Operating in the saturation region.
Large-signal equivalent-circuit model of an p-channel MOSFET : Operating in the saturation region.
Large Signal Model : MOSFET
Transfer characteristic of an amplifier
Conceptual circuit utilized to study the operation of the MOSFET as a small-signal amplifier.
The DC BIAS POINT
To Ensure Saturation-region Operation
Signal Current in Drain Terminal
Total instantaneous voltages vGS and vD
Small-signal ‘π’ models for the MOSFET
Common Source amplifier circuitExample 4-10
Small Signal ‘T’ Model : NMOSFET
Small Signal Models
‘T’ Model
Single Stage MOS Amplifier
Amplifiers Configurations
Common Source Amplifier (CS) :Configuration
Common Source Amplifier (CS)• Most widely used
• 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)
Small Signal Hybrid “π” Model : (CS)
Gin RR sig
sigG
Ggs v
RR
Rv
LDogsmo RRrvgv ||||
sigG
GLDom
gs
ov RR
RRRrg
v
vG ||||
Doo Rr ||R
sig
gs
gs
o
sig
ov v
v
v
v
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
• 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
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 & effect of using ro in model would be studied in Chapter ‘6’
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
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
4. CG has much higher output Resistance5. CG is unity current Gain amplifier or a Current Buffer6. CG has superior High Frequency Response.
A common-drain or source-follower amplifier.
Small-signal equivalent-circuit model
Small-signal Analysis : CD
(a) A common-drain or source-follower amplifier :output resistance Rout of the source follower.
mmoout ggrR
11||
(a) 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
Common Drain Circuit (CD) Source Follower
Summary & Comparison
Small Signal Model
Figure P6.123
Small Signal Model
VBE=0.7 Vβ =200K’n(W/L)=2mA/V2
Vt=1V
Figure P6.123
DC Analysis