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Small Signal Model MOS Field-Effect Transistors (MOSFETs) PowerPoint PPT Presentation


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Small Signal Model MOS Field-Effect Transistors (MOSFETs). 1. Quiz No 3 DE 27 (CE). 20-03-07. R out. Draw small signal model (4) Find expression for R out (2) Prove v o /v sig = ( β 1 α 2 R C )/(R sig +r π ) (4).

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Small Signal Model MOS Field-Effect Transistors (MOSFETs)

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Small Signal Model

MOS Field-Effect Transistors (MOSFETs)

1


Quiz No 3 DE 27 (CE)

20-03-07

Rout.

  • Draw small signal model (4)

  • Find expression for Rout(2)

  • Prove vo/vsig = (β1α2RC)/(Rsig+rπ)(4).


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 vGSand vD


Small-signal ‘π’ models for the MOSFET


Common Source amplifier circuit

Example 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 & voltagescoupling 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)


Small-signal analysis performed directly on the amplifier circuit with the MOSFET model implicitly utilized.


Common Source Amplifier (CS) Summary

  • Input Resistance is infinite (Ri=∞)

  • Output Resistance = RD

  • Voltage Gain is substantial


Common-source amplifier

with a resistance RSin 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.


Small-signal Analysis.


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


A small-signal Analusis : CG


Small signal analysis directly on circuit


The common-gate amplifier fed with a current-signal input.


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.


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.


(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


Quiz No 4

27-03-07

  • Draw/Write the Following:


Problem 5-44


SOLUTION : DC Analysis


SOLUTION : DC Analysis

IE

Check for Active Mode

IB


Solution Small Signal Analysis


Solution Small Signal Analysis


Solution Small Signal Analysis : Input Resistance

ib

+

vb

-

Rin


Solution Small Signal Analysis : Output Resistance

Itest

IE

IRC

IE/(1+ß)

Rout


Solution Small Signal Analysis : Voltage Gain

+

veb

-

-

Vo

+

+

vi

-


Solution Small Signal Analysis : Voltage gain

+

veb

-

+

vi

-


Solution Small Signal Analysis : Voltage Gain

+

vi

-


Solution Small Signal Analysis : Voltage Gain


Solution Small Signal Analysis : Voltage Gain

-

Vo

+

+

vi

-


Problem


Small Signal Model MOSFET : CD


Solution Small Signal Analysis

1/gm

D

gmvsg


Solution Small Signal Analysis : Input Resistance

1/gm

D

gmvsg

Ig=0

Rin


1/gm

D

gmvsg

Solution Small Signal Analysis : Output Resistance

Itest

ID

IRD

Vtest

IG=0

Rout


Solution Small Signal Analysis : Voltage Gain

1/gm

D

gmvsg

+

vsg

-

-

+

+

vi

-


Solution Small Signal Analysis : Voltage gain

1/gm

D

gmvsg

+

vsg

-

+

vi

-


Solution Small Signal Analysis : Voltage Gain

+

vi

-


Solution Small Signal Analysis : Voltage Gain


Solution Small Signal Analysis : Voltage Gain

-

+

+

vi

-


Solution Small Signal Analysis


Problem 6-127(e)


DC Analysis 6-127(e)


Small Signal Model


Small Signal Model


Small Signal Model

Rin

Rout


Problem6-127(f)Replacing BJT with MOSFET


Small Signal Model


Small Signal Model


Small Signal Model

Rin

Rout


Problem 6-127(f)


Solution P6-127(f)

+

vbe2

-

+

veb1

-


+

vbe2

+

-

+

vi

veb1

-

-

Solution P6-127(f)


Problem 6-127(f) with MOSFET


Solution P6-127(f)

+

vgs2

-

+

vsg1

-


+

vgs2

-

+

vsg1

-

Solution P6-127(f)

ig1=0

+

vi

-


Comparison BJT/MOSFET Cct


Small Signal Model


Problem 6-123

VBE=0.7 V

β =200

K’n(W/L)=2mA/V2

Vt=1V

Figure P6.123


DC Analysis

Figure P6.123


VBE=0.7 V

β =200

K’n(W/L)=2mA/V2

Vt1=1V

Vt2=25mV

DC Analysis

1mA

2V

IG=0

0.7V

I=0.7/6.8=0.1mA


Small Signal Model


Small Signal Model


Small Signal Model : Voltage Gain

ig=0

+

+

vi

vbe2

-

-


Small Signal Model : Input Resistance

ii

ig=0

+

vi

-

Rin


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