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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A traveler once came to the mosque to see the
prophet.After greeting the prophet he was asked
where he was from. The traveler replied that he
came from very farjust to get a few questions
answered. Following is thedialogue between the
traveler and the prophet.Traveler I do not
want adhaab to be written in my account.
Prophet behave well with your
parentsTraveler I want to be known amongst
people as anintelligent person. Prophet fear
Allah always.Traveler I want to be counted
amongst Allah's favorites.Prophet recite Quran
every morning and evening. Traveler I want my
heart to always be enlightened.(roshan and
munawer) Prophet never forget death
4
Traveler I never want to be away from Allah's
blessing. Prophet always treat fellow creatures
well.Traveler I never want to be harmed by my
enemies. Prophet always have faith in only
Allah.Traveler I never want to be humiliated.
Prophet becareful of your actions.Traveler I
wish to live long. Prophet always do sila-rahm.
(Goodness towards blood relations)Traveler I
want my sustenance to increase. Prophet always
be in wudhoo.Traveler I wish to stay free of
adhaab in the grave.Prophet always wear pure
(paak) clothes. Traveler I never want to burn
in hell.Prophet control your eyes and tongue.
5
Traveler how do I get my sins forgiven?
Prophet always ask forgiveness from Allah with
a lot of humility.Traveler I want people to
respect me always. Prophet never extend your
hands of need at people.Traveler I want to
always be honored. Prophet never humiliate or
put down anyone.Traveler I don't want to be
squeezed by fishare qabr.(Squeezing in the grave)
Prophet recite sura e mulk often.Traveler I
want my wealth to increase. Prophet recite sura
e waqia every night.Traveler I want to be safe
and at peace on day of judgment. Prophet do
zikr of Allah from dusk to night.Traveler I
want to be in full attention and concentration
during namaaz. Prophet always do wudhoo with
concentration and attention
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Enhancement-type NMOS transistor
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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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Small Signal Model
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Small Signal Model
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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