Small Signal Model

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Small Signal Model MOS Field-Effect Transistors
(MOSFETs)
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Quiz No 3 DE 27 (CE)
20-03-07
Rout.

1. Draw small signal model (4)
2. Find expression for Rout (2)
3. Prove vo/vsig (ß1a2RC)/(Rsigrp) (4).

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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.
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Enhancement-type NMOS transistor
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MOSFET Analysis
iD iS, iG 0
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Large-signal equivalent-circuit model of an
n-channel MOSFET Operating in the saturation
region.
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Large-signal equivalent-circuit model of an
p-channel MOSFET Operating in the saturation
region.
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Large Signal Model MOSFET
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Transfer characteristic of an amplifier
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Conceptual circuit utilized to study the
operation of the MOSFET as a small-signal
amplifier.
The DC BIAS POINT
To Ensure Saturation-region Operation
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Signal Current in Drain Terminal
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Figure 4.35 Small-signal operation of the
enhancement MOSFET amplifier.
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Total instantaneous voltages vGS and vD
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Small-signal p models for the MOSFET
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Common Source amplifier circuit Example 4-10
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Small Signal T Model NMOSFET
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Small Signal Models
T Model
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Single Stage MOS Amplifier
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Amplifiers Configurations
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Common Source Amplifier (CS) Configuration
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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

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Small Signal Hybrid p Model (CS)
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Small Signal Hybrid p Model (CS)
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Small-signal analysis performed directly on the
amplifier circuit with the MOSFET model
implicitly utilized.
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Common Source Amplifier (CS) Summary

• Input Resistance is infinite (Ri8)
• Output Resistance RD
• Voltage Gain is substantial

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Common-source amplifier with a resistance RS in
the source lead
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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

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Small-signal equivalent circuit with ro neglected.
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Small-signal Analysis.
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Voltage Gain CS with RS
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• Source Resistance can be used to control the
  magnitude of the signal vgs thus ensure that
  vgs does not become too large to cause non-linear
  distortion
• vgs ltlt 2(VGS-Vt) ltlt VOV

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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
• (1gmRs) Amount of feedback
• Rs is called Source degeneration resistance as it
  reduces the gain

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Small-signal equivalent circuit directly on
Circuit
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A common-gate amplifier based on the circuit
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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

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A small-signal equivalent circuit
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A small-signal Analusis CG
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A small-signal Analusis CG
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Small signal analysis directly on circuit
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The common-gate amplifier fed with a
current-signal input.
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Summary CG

• 4. CG has much higher output Resistance
• CG is unity current Gain amplifier or a Current
  Buffer
• CG has superior High Frequency Response.

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Common Gate

• Rin in independent of RL Rin 1/gm gm in
  order of mA/V.
• Input resistance of the CG Amplifier is
  relatively low (in order of 1kv) than CS
  Amplifier
• Loss of signal
• CG is acts as Unity gain current amplifier
  current buffer useful for a Cascade circuitry

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A common-drain or source-follower amplifier.
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Small-signal equivalent-circuit model
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Small-signal Analysis CD
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(a) A common-drain or source-follower amplifier
output resistance Rout of the source follower.
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(a) A common-drain or source-follower amplifier.
Small-signal analysis performed directly on the
circuit.
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Common Source Circuit (CS)
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Common Source Circuit (CS) With RS
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Common Gate Circuit (CG) Current Follower
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Common Drain Circuit (CD) Source Follower
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Summary Comparison
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Quiz No 4
27-03-07

• Draw/Write the Following

BJT BJT MOSFET MOSFET
Types npn pnp nMOS pMOS
Symbols
p Model
T Model
gm
Re/rs
rp/rg

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(No Transcript)
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(No Transcript)
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Problem 5-44
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SOLUTION DC Analysis
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SOLUTION DC Analysis
IE
Check for Active Mode
IB
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Solution Small Signal Analysis
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Solution Small Signal Analysis
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Solution Small Signal Analysis Input Resistance
ib

vb
-
Rin
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Solution Small Signal Analysis Output Resistance
Itest
IE
IRC
IE/(1ß)
Rout
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Solution Small Signal Analysis Voltage Gain

veb
-
-
Vo


vi
-
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Solution Small Signal Analysis Voltage gain

veb
-

vi
-
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Solution Small Signal Analysis Voltage Gain

vi
-
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Solution Small Signal Analysis Voltage Gain
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Solution Small Signal Analysis Voltage Gain
-
Vo


vi
-
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Problem
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Small Signal Model MOSFET CD
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Solution Small Signal Analysis
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Solution Small Signal Analysis Input Resistance
Ig0
Rin
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Solution Small Signal Analysis Output Resistance
Itest
ID
IRD
Vtest
IG0
Rout
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Solution Small Signal Analysis Voltage Gain

vsg
-
-


vi
-
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Solution Small Signal Analysis Voltage gain

vsg
-

vi
-
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Solution Small Signal Analysis Voltage Gain

vi
-
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Solution Small Signal Analysis Voltage Gain
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Solution Small Signal Analysis Voltage Gain
-


vi
-
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Solution Small Signal Analysis
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Problem 6-127(e)
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Transistor Pairings Amplifiers

• Common Emitter Common Base (CE-CB)
• Common Emitter Common Collector (CE-CC)
• Common Collector - Common Emitter (CC-CE)
• Common Collector - Common Base (CC-CB)

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DC Analysis 6-127(e)
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Small Signal Model
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Small Signal Model
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Small Signal Model
Rin
Rout
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Problem6-127(f)Replacing BJT with MOSFET
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Small Signal Model
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Small Signal Model
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Small Signal Model
Rin
Rout
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(No Transcript)
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Problem 6-127(f)
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Solution P6-127(f)

vbe2
-

veb1
-
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Solution P6-127(f)
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Problem 6-127(f) with MOSFET
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Solution P6-127(f)

vgs2
-

vsg1
-
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Solution P6-127(f)
ig10

vi
-
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Comparison BJT/MOSFET Cct
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Small Signal Model
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Problem 6-123
VBE0.7 V ß 200 Kn(W/L)2mA/V2 Vt1V
Figure P6.123
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DC Analysis
Figure P6.123
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DC Analysis
VBE0.7 V ß 200 Kn(W/L)2mA/V2 Vt11V Vt225mV
1mA
2V
0.7V
IG0
I0.7/6.80.1mA
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Small Signal Model
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Small Signal Model
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Small Signal Model Voltage Gain
ig0


vi
vbe2
-
-
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Small Signal Model Input Resistance
ii
ig0

vi
-
Rin