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Title: Small Signal Model


1
Small Signal Model MOS Field-Effect Transistors
(MOSFETs)
1
2
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).

3
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
6
Enhancement-type NMOS transistor
7
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.
8
Enhancement-type NMOS transistor
9
MOSFET Analysis
iD iS, iG 0
10
Large-signal equivalent-circuit model of an
n-channel MOSFET Operating in the saturation
region.
11
Large-signal equivalent-circuit model of an
p-channel MOSFET Operating in the saturation
region.
12
Large Signal Model MOSFET
13
Transfer characteristic of an amplifier
14
Conceptual circuit utilized to study the
operation of the MOSFET as a small-signal
amplifier.
The DC BIAS POINT
To Ensure Saturation-region Operation
15
Signal Current in Drain Terminal
16
Figure 4.35 Small-signal operation of the
enhancement MOSFET amplifier.
17
Total instantaneous voltages vGS and vD
18
Small-signal p models for the MOSFET
19
Common Source amplifier circuit Example 4-10
20
Small Signal T Model NMOSFET
21
Small Signal Models
T Model
22
Single Stage MOS Amplifier
23
Amplifiers Configurations
24
Common Source Amplifier (CS) Configuration
25
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

26
Small Signal Hybrid p Model (CS)
27
Small Signal Hybrid p Model (CS)
28
Small-signal analysis performed directly on the
amplifier circuit with the MOSFET model
implicitly utilized.
29
Common Source Amplifier (CS) Summary
  • Input Resistance is infinite (Ri8)
  • Output Resistance RD
  • Voltage Gain is substantial

30
Common-source amplifier with a resistance RS in
the source lead
31
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

32
Small-signal equivalent circuit with ro neglected.
33
Small-signal Analysis.
34
Voltage Gain CS with RS
35
  • 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

36
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

37
Small-signal equivalent circuit directly on
Circuit
38
A common-gate amplifier based on the circuit
39
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

40
A small-signal equivalent circuit
41
A small-signal Analusis CG
42
A small-signal Analusis CG
43
Small signal analysis directly on circuit
44
The common-gate amplifier fed with a
current-signal input.
45
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.

46
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

47
A common-drain or source-follower amplifier.
48
Small-signal equivalent-circuit model
49
Small-signal Analysis CD
50
(a) A common-drain or source-follower amplifier
output resistance Rout of the source follower.
51
(a) A common-drain or source-follower amplifier.
Small-signal analysis performed directly on the
circuit.
52
Common Source Circuit (CS)
53
Common Source Circuit (CS) With RS
54
Common Gate Circuit (CG) Current Follower
55
Common Drain Circuit (CD) Source Follower
56
Summary Comparison
57
Small Signal Model
58
Small Signal Model
VBE0.7 V ß 200 Kn(W/L)2mA/V2 Vt1V
Figure P6.123
59
DC Analysis
Figure P6.123
60
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