UNIT III

JFET and MOSFET Amplifiers

OUTLINE

• Small signal Analysis of JFET amplifiers• Small signal Analysis of MOSFET and JFET• BICMOS cascode Amplifier

FET ( Field Effect Transistor)

• Unipolar device i. e. operation depends on only one type of charge carriers.

• Voltage controlled Device (gate voltage controls drain current).

• Very high input impedance (109-1012 ).

• Source and drain are interchangeable in most Low-frequency applications.

• Low-power consumption.

• Less Noisy as Compared to BJT.• Very small in size, occupies very small space in

Ics.

Types of Field Effect Transistors (The Classification)

» JFET

MOSFET (IGFET)

n-Channel JFET

p-Channel JFET

n-Channel EMOSFET

p-Channel EMOSFET

Enhancement MOSFET

Depletion MOSFET

n-Channel DMOSFET

p-Channel DMOSFET

FET

Figure: n-Channel JFET.

The Junction Field Effect Transistor (JFET)

JFET

• There are two basic configurations of junction field effect transistor, the N-channel JFET and the P-channel JFET.

• The N-channel JFET’s channel is doped with donor impurities meaning that the flow of current through the channel is negative (hence the term N-channel) in the form of electrons.

Gate

Drain

Source

SYMBOLS

n-channel JFET

Gate

Drain

Source

n-channel JFETOffset-gate symbol

Gate

Drain

Source

p-channel JFET

Small signal Analysis

• The coupling capacitors bypass capacitor are

short circuitED

• Short the DC supply voltage

• Replace the FET with the hybrid-p model

Common source JFET

Small signal -Common source JFET-contd…

Small signal -Common source JFET-contd…

Source Follower JFET

Small signal equivalent-Source Follower JFET

Av= gmRs/(1+gmRs)

Common Gate JFET

Small signal model-Common Gate JFET

AV= -gmRd Rin=1/gm

Self bias of JFET

Small signal equivalent -Self bias of JFET-contd…

Enhancement MOSFET showing channel length L and channel width W.

MOSFET- SYMBOL

Small-signal equivalent circuit for FETs.

FET small-signal equivalent circuit that accounts for the dependence of iD on vDS.

MOSFET Characteristics

Common source Amplifier -MOSFET

For drawing an a c equivalent circuit of Amp.

•Assume all Capacitors C1, C2, Cs as short circuit elements for ac signal

•Short circuit the d c supply

•Replace the FET by its small signal model

Small signal -Common source Amplifier -MOSFET

Analysis of CS Amplifier

A C Equivalent Circuit

Simplified A C Equivalent Circuit

Analysis of CS Amplifier-contd…

LgsmLoo

gs

ov

RvgRiv

v

vA

gain, Voltage

21 imp., Input RRRZ

Gin

dDLLmgs

ov

rRRRgv

vA ,

Dd

DdDdo Rr

RrRrZ

imp., put Out

Small Signal ‘T’ Model : NMOSFET

Small Signal Models

‘T’ Model

Analysis of CS Amplifier with Potential Divider Bias

)R||(rgAv Ddm

DR10r D,m

dRgAv

)R||(rgAv Ddm

This is a CS amplifier configuration therefore the input is on the gate and the output is on the drain. 21 R||RZi

Dd R||rZo

DdD 10RrRZo

An Amplifier Circuit using MOSFET(CS Amp.)

A small signal equivalent circuit of CS Amp.

Common Source Amplifier (CS)

• 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)

sigsigG

Ggs v

RR

Rv

sig

gs

gs

o

sig

ov v

v

v

v

v

vG

Small Signal Hybrid “π” Model : (CS)

Gin RR

Doo Rr ||R

LDogsmo RRrvgv ||||

sigG

GLDom

gs

ov RR

RRRrg

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

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

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

BJT / MOSFET

LCosigB

Bm

sig

o

Coout

Bin

RRrRrR

rRg

v

v

RrR

rRR

||||||

||

||

||

LDosigG

Gm

sig

o

Doout

Gin

RRrRR

Rg

v

v

RrR

RR

||||

||

1,

• 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

MOSFET -Source follower.

Small-signal ac equivalent circuit for the source follower.

Equivalent circuit used to find the output resistance of the source follower.

Common-gate amplifier.

Small signal-Common Gate

Small-signal ‘π’ models for the MOSFET

Voltage swing limitation

• Up swing limited by transistor going in to cut off• Vout (max)= VDD• Lower swing limited by MOSFET entering in to

linear region• Vou(min)-VGG-VT

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

• 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 common-drain or source-follower amplifier :output resistance Rout of the source follower.

mmoout ggrR

11||

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

Small Signal Model MOSFET : CD

Small Signal Analysis CD

1/gm

gmvsgD

1/gm

gmvsgD

Solution Small Signal Analysis : Input Resistance

Rin

Ig=0

inR

1/gm

gmvsgD

Solution Small Signal Analysis : Output ResistanceItest

ID

IG=0

IRD

Rout

test

testout I

VR

mD

m

test

D

test

testout g

R

gV

RVV

R1

||

/1

DRtest IIIC

m

testD

g

VI

1

D

testR R

VI

D

Vtest0 V

1/gm

gmvsgD

Solution Small Signal Analysis : Voltage Gain

+

- LDm

sg

o RRgv

v||+

-

vi

+

-

vsgsig

i

i

sg

sg

o

sig

o

v

v

v

v

v

v

v

v

vo

1/gm

gmvsgD

Solution Small Signal Analysis : Voltage gain

+

-

vi

+

-

vsg

LDm

m

i

sg

RRg

g

v

v

||1

1

1/gm

gmvsgD

Solution Small Signal Analysis : Voltage Gain

+

-

vi

inRsigi vv

Solution Small Signal Analysis : Voltage Gain

)R(Rg

g)||R(Rg

v

v

LDm

mLDm

sig

o

||1

1

sigi vv

LDmsg

o RRgv

v||

sig

i

i

sg

sg

o

sig

o

v

v

v

v

v

v

v

v

LDm

m

i

sg

RRg

g

v

v

||1

1

)R(Rg

)||R(R

v

v

LDm

LD

sig

o

||1

Then

1/gm

gmvsgD

Solution Small Signal Analysis : Voltage Gain

+

- LD

m

LD

i

o

RRg

RR

v

v

||1||

+

-

vi

sig

i

i

o

sig

o

v

v

v

v

v

v

sigi vv

LD

m

LD

sig

o

RRg

RR

v

v

||1||

BICMOS

BICMOS -contd…

• Combining the high gain of BJT and infinite impedance of MOSFET will lead to BiCMOS differential amplifier design.

• Rs = typical 100 KW

• BICMOS cascode amplifier has overall voltage gain of C-S, but with frequency response comparable to CB Amplifier.

BICMOS -contd…

• The basic idea is to combine the high Rin and large transconductance (g m) of a commonsource (common-emitter) amplifier with the current-buffering property and superior high-frequency response of the common-gate (common-base) circuit

BICMOS -contd…

• In response to input signal voltage vi, the CS transistor Q1 conducts a current signal gm1 vi in its drain terminal and feeds it to the source of the CG transistor Q 2 (cascode transistor).

• Q 2 passes signal to its drain and to the load RL. Q 2 acts as a buffer, presenting low Rin to the drain of Q1 and providing high Rout at output.