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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

Small Signal Model

MOS Field-Effect Transistors (MOSFETs)

1


Quiz no 3 de 27 ce

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).


Small signal model mos field effect transistors mosfets

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.


Small signal model mos field effect transistors mosfets

Enhancement-type NMOS transistor:


Mosfet analysis

MOSFET Analysis

iD = iS, iG = 0


Small signal model mos field effect transistors mosfets

Large-signal equivalent-circuit model of an n-channel MOSFET : Operating in the saturation region.


Small signal model mos field effect transistors mosfets

Large-signal equivalent-circuit model of an p-channel MOSFET : Operating in the saturation region.


Small signal model mos field effect transistors mosfets

Large Signal Model : MOSFET


Small signal model mos field effect transistors mosfets

Transfer characteristic of an amplifier


Small signal model mos field effect transistors mosfets

Conceptual circuit utilized to study the operation of the MOSFET as a small-signal amplifier.

The DC BIAS POINT

To Ensure Saturation-region Operation


Small signal model mos field effect transistors mosfets

Signal Current in Drain Terminal


Small signal model mos field effect transistors mosfets

Total instantaneous voltages vGSand vD


Small signal model mos field effect transistors mosfets

Small-signal ‘π’ models for the MOSFET


Small signal model mos field effect transistors mosfets

Common Source amplifier circuit

Example 4-10


Small signal model mos field effect transistors mosfets

Small Signal ‘T’ Model : NMOSFET


Small signal models

Small Signal Models

‘T’ Model


Single stage mos amplifier

Single Stage MOS Amplifier


Small signal model mos field effect transistors mosfets

Amplifiers Configurations


Common source amplifier cs configuration

Common Source Amplifier (CS) :Configuration


Common source amplifier cs

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 model cs

Small Signal Hybrid “π” Model (CS)


Small signal hybrid model cs1

Small Signal Hybrid “π” Model : (CS)


Small signal model mos field effect transistors mosfets

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


Common source amplifier cs summary

Common Source Amplifier (CS) Summary

  • Input Resistance is infinite (Ri=∞)

  • Output Resistance = RD

  • Voltage Gain is substantial


Small signal model mos field effect transistors mosfets

Common-source amplifier

with a resistance RSin the source lead


The common source amplifier with a source resistance

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 model mos field effect transistors mosfets

Small-signal equivalent circuit with ro neglected.


Small signal model mos field effect transistors mosfets

Small-signal Analysis.


Small signal model mos field effect transistors mosfets

Voltage Gain : CS with RS


Common source configuration with r s

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 model mos field effect transistors mosfets

Small-signal equivalent circuit directly on Circuit


Small signal model mos field effect transistors mosfets

A common-gate amplifier based on the circuit


Common gate cg amplifier

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


Small signal model mos field effect transistors mosfets

A small-signal equivalent circuit


Small signal model mos field effect transistors mosfets

A small-signal Analusis : CG


Small signal model mos field effect transistors mosfets

A small-signal Analusis : CG


Small signal model mos field effect transistors mosfets

Small signal analysis directly on circuit


Small signal model mos field effect transistors mosfets

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


Summary cg

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.


Small signal model mos field effect transistors mosfets

A common-drain or source-follower amplifier.


Small signal model mos field effect transistors mosfets

Small-signal equivalent-circuit model


Small signal model mos field effect transistors mosfets

Small-signal Analysis : CD


Small signal model mos field effect transistors mosfets

(a) A common-drain or source-follower amplifier :output resistance Rout of the source follower.


Small signal model mos field effect transistors mosfets

(a) A common-drain or source-follower amplifier. : Small-signal analysis performed directly on the circuit.


Small signal model mos field effect transistors mosfets

Common Source Circuit (CS)


Small signal model mos field effect transistors mosfets

Common Source Circuit (CS) With RS


Small signal model mos field effect transistors mosfets

Common Gate Circuit (CG)

Current Follower


Small signal model mos field effect transistors mosfets

Common Drain Circuit (CD)

Source Follower


Summary comparison

Summary & Comparison


Quiz no 4

Quiz No 4

27-03-07

  • Draw/Write the Following:


Small signal model mos field effect transistors mosfets

Problem 5-44


Small signal model mos field effect transistors mosfets

SOLUTION : DC Analysis


Small signal model mos field effect transistors mosfets

SOLUTION : DC Analysis

IE

Check for Active Mode

IB


Small signal model mos field effect transistors mosfets

Solution Small Signal Analysis


Small signal model mos field effect transistors mosfets

Solution Small Signal Analysis


Small signal model mos field effect transistors mosfets

Solution Small Signal Analysis : Input Resistance

ib

+

vb

-

Rin


Solution small signal analysis output resistance

Solution Small Signal Analysis : Output Resistance

Itest

IE

IRC

IE/(1+ß)

Rout


Small signal model mos field effect transistors mosfets

Solution Small Signal Analysis : Voltage Gain

+

veb

-

-

Vo

+

+

vi

-


Small signal model mos field effect transistors mosfets

Solution Small Signal Analysis : Voltage gain

+

veb

-

+

vi

-


Small signal model mos field effect transistors mosfets

Solution Small Signal Analysis : Voltage Gain

+

vi

-


Small signal model mos field effect transistors mosfets

Solution Small Signal Analysis : Voltage Gain


Small signal model mos field effect transistors mosfets

Solution Small Signal Analysis : Voltage Gain

-

Vo

+

+

vi

-


Small signal model mos field effect transistors mosfets

Problem


Small signal model mos field effect transistors mosfets

Small Signal Model MOSFET : CD


Small signal model mos field effect transistors mosfets

Solution Small Signal Analysis

1/gm

D

gmvsg


Small signal model mos field effect transistors mosfets

Solution Small Signal Analysis : Input Resistance

1/gm

D

gmvsg

Ig=0

Rin


Solution small signal analysis output resistance1

1/gm

D

gmvsg

Solution Small Signal Analysis : Output Resistance

Itest

ID

IRD

Vtest

IG=0

Rout


Small signal model mos field effect transistors mosfets

Solution Small Signal Analysis : Voltage Gain

1/gm

D

gmvsg

+

vsg

-

-

+

+

vi

-


Small signal model mos field effect transistors mosfets

Solution Small Signal Analysis : Voltage gain

1/gm

D

gmvsg

+

vsg

-

+

vi

-


Small signal model mos field effect transistors mosfets

Solution Small Signal Analysis : Voltage Gain

+

vi

-


Small signal model mos field effect transistors mosfets

Solution Small Signal Analysis : Voltage Gain


Small signal model mos field effect transistors mosfets

Solution Small Signal Analysis : Voltage Gain

-

+

+

vi

-


Small signal model mos field effect transistors mosfets

Solution Small Signal Analysis


Problem 6 127 e

Problem 6-127(e)


Dc analysis 6 127 e

DC Analysis 6-127(e)


Small signal model

Small Signal Model


Small signal model mos field effect transistors mosfets

Small Signal Model


Small signal model mos field effect transistors mosfets

Small Signal Model

Rin

Rout


Problem6 127 f replacing bjt with mosfet

Problem6-127(f)Replacing BJT with MOSFET


Small signal model1

Small Signal Model


Small signal model2

Small Signal Model


Small signal model mos field effect transistors mosfets

Small Signal Model

Rin

Rout


Problem 6 127 f

Problem 6-127(f)


Solution p6 127 f

Solution P6-127(f)

+

vbe2

-

+

veb1

-


Solution p6 127 f1

+

vbe2

+

-

+

vi

veb1

-

-

Solution P6-127(f)


Problem 6 127 f with mosfet

Problem 6-127(f) with MOSFET


Solution p6 127 f2

Solution P6-127(f)

+

vgs2

-

+

vsg1

-


Solution p6 127 f3

+

vgs2

-

+

vsg1

-

Solution P6-127(f)

ig1=0

+

vi

-


Comparison bjt mosfet cct

Comparison BJT/MOSFET Cct


Small signal model3

Small Signal Model


Problem 6 123

Problem 6-123

VBE=0.7 V

β =200

K’n(W/L)=2mA/V2

Vt=1V

Figure P6.123


Dc analysis

DC Analysis

Figure P6.123


Small signal model mos field effect transistors mosfets

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 model4

Small Signal Model


Small signal model5

Small Signal Model


Small signal model voltage gain

Small Signal Model : Voltage Gain

ig=0

+

+

vi

vbe2

-

-


Small signal model input resistance

Small Signal Model : Input Resistance

ii

ig=0

+

vi

-

Rin


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