1

Small Signal ModelMOS Field-Effect Transistors (MOSFETs)

Quiz No 3 DE 27 (CE)

(a) Draw small signal model (4)(b) Find expression for Rout (2) (c) Prove vo/vsig = (β1α2RC)/(Rsig+rπ) (4).

Rout.

20-03-07

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After greeting the prophet he was asked where he was

from. The traveler replied that he came from very far

just to get a few questions answered. Following is the

dialogue between the traveler and the prophet.

Traveler: I do not want adhaab to be written in my account.

Prophet: behave well with your parents

Traveler: I want to be known amongst people as an

intelligent 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

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.

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

Enhancement-type NMOS transistor:

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 vGS and vD

Small-signal ‘π’ models for the MOSFET

Common Source amplifier circuitExample 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 & 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

Small Signal Hybrid “π” Model (CS)

Small Signal Hybrid “π” Model : (CS)

Gin RR sig

sigG

Ggs v

RR

Rv

LDogsmo RRrvgv ||||

sigG

GLDom

gs

ov RR

RRRrg

v

vG ||||

Doo Rr ||R

sig

gs

gs

o

sig

ov v

v

v

v

v

vG

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

Gin RR

sigG

GLDom

gs

o

RR

RRRrg

v

v||||

Doo Rr ||R

• Input Resistance is infinite (Ri=∞)

• Output Resistance = RD

• Voltage Gain is substantial

Common Source Amplifier (CS) Summary

Gin RR

sigG

GLDom

gs

o

RR

RRRrg

v

v||||

Doo Rr ||R

Common-source amplifier with a resistance RS in 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.

Sm

g

Rg

vi

1

Do

Gin

RR

RR

Small-signal Analysis.

sig

i

i

gs

gs

o

sig

ov v

v

v

v

v

v

v

vG

Sm

LDm

sigG

Gv

sig

o

sigsigG

Gi

Sm

ii

Sm

mgs

LDgsmo

Rg

RRg

RR

RG

v

v

vRR

Rv

Rg

vv

Rg

gv

RRvgv

1

||

11

1

||

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

mim

i

i

iin gvg

v

i

vR

1

Dout RR

A small-signal Analusis : CG

sigm

LDm

sig

ov

sigm

sigsig

sigm

msig

sigin

ini

LDimo

sig

i

i

o

sig

ov

Rg

RRg

v

vG

Rg

vv

Rg

gv

RR

Rv

RRvgv

v

v

v

v

v

vG

1

||

11

1

||

Small signal analysis directly on circuit

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

Summary : CG

4. CG has much higher output Resistance5. CG is unity current Gain amplifier or a Current Buffer6. 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.

mmoout ggrR

11||

(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

Small Signal Model

Figure P6.123

Small Signal Model

VBE=0.7 Vβ =200K’n(W/L)=2mA/V2

Vt=1V

Figure P6.123

DC Analysis