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

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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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.

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.

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 MOSFET ‘π’ models for the MOSFET

Example 4-10

Small Signal Models MOSFET

‘T’ Model

### Single Stage MOS Amplifier MOSFET

Common Source Amplifier (CS) : MOSFET Configuration

• 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 “ MOSFET π” Model (CS)

Small Signal Hybrid “ MOSFET π” Model : (CS)

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

Common Source Amplifier (CS) Summary circuit with the MOSFET model implicitly utilized.

• Input Resistance is infinite (Ri=∞)

• Output Resistance = RD

• Voltage Gain is substantial

Common-source amplifier circuit with the MOSFET model implicitly utilized.

with a resistance RSin the source lead

The Common Source Amplifier with a Source Resistance circuit with the MOSFET model implicitly utilized.

• 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 circuit with the MOSFET model implicitly utilized.ro neglected.

Small-signal Analysis. circuit with the MOSFET model implicitly utilized.

Voltage Gain : CS with R circuit with the MOSFET model implicitly utilized.S

Common Source Configuration with R circuit with the MOSFET model implicitly utilized.s

• 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 circuit with the MOSFET model implicitly utilized.

A common-gate amplifier based on the circuit circuit with the MOSFET model implicitly utilized.

Common Gate (CG) Amplifier circuit with the MOSFET model implicitly utilized.

• 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 circuit with the MOSFET model implicitly utilized.

A small-signal Analusis : CG circuit with the MOSFET model implicitly utilized.

A small-signal Analusis : CG circuit with the MOSFET model implicitly utilized.

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

The common-gate amplifier fed with a current-signal input. circuit with the MOSFET model implicitly utilized.

Summary : CG circuit with the MOSFET model implicitly utilized.

• 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. circuit with the MOSFET model implicitly utilized.

Small-signal equivalent-circuit model circuit with the MOSFET model implicitly utilized.

Small-signal Analysis : CD circuit with the MOSFET model implicitly utilized.

(a) circuit with the MOSFET model implicitly utilized. A common-drain or source-follower amplifier :output resistance Rout of the source follower.

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

Common Source Circuit (CS) circuit with the MOSFET model implicitly utilized.

Common Source Circuit (CS) With R circuit with the MOSFET model implicitly utilized.S

Common Gate Circuit (CG) circuit with the MOSFET model implicitly utilized.

Current Follower

Common Drain Circuit (CD) circuit with the MOSFET model implicitly utilized.

Source Follower

Summary & Comparison circuit with the MOSFET model implicitly utilized.

Quiz No 4 circuit with the MOSFET model implicitly utilized.

27-03-07

• Draw/Write the Following:

Problem 5-44 circuit with the MOSFET model implicitly utilized.

SOLUTION : DC Analysis circuit with the MOSFET model implicitly utilized.

SOLUTION : DC Analysis circuit with the MOSFET model implicitly utilized.

IE

Check for Active Mode

IB

Solution Small Signal Analysis circuit with the MOSFET model implicitly utilized.

Solution Small Signal Analysis circuit with the MOSFET model implicitly utilized.

Solution Small Signal Analysis : circuit with the MOSFET model implicitly utilized.Input Resistance

ib

+

vb

-

Rin

Solution Small Signal Analysis : circuit with the MOSFET model implicitly utilized.Output Resistance

Itest

IE

IRC

IE/(1+ß)

Rout

Solution Small Signal Analysis : circuit with the MOSFET model implicitly utilized.Voltage Gain

+

veb

-

-

Vo

+

+

vi

-

Solution Small Signal Analysis : circuit with the MOSFET model implicitly utilized.Voltage gain

+

veb

-

+

vi

-

Solution Small Signal Analysis : circuit with the MOSFET model implicitly utilized.Voltage Gain

+

vi

-

Solution Small Signal Analysis : circuit with the MOSFET model implicitly utilized.Voltage Gain

Solution Small Signal Analysis : circuit with the MOSFET model implicitly utilized.Voltage Gain

-

Vo

+

+

vi

-

Problem circuit with the MOSFET model implicitly utilized.

Small Signal Model MOSFET : CD circuit with the MOSFET model implicitly utilized.

Solution Small Signal Analysis circuit with the MOSFET model implicitly utilized.

1/gm

D

gmvsg

Solution Small Signal Analysis : circuit with the MOSFET model implicitly utilized.Input Resistance

1/gm

D

gmvsg

Ig=0

Rin

1/g circuit with the MOSFET model implicitly utilized.m

D

gmvsg

Solution Small Signal Analysis : Output Resistance

Itest

ID

IRD

Vtest

IG=0

Rout

Solution Small Signal Analysis : circuit with the MOSFET model implicitly utilized.Voltage Gain

1/gm

D

gmvsg

+

vsg

-

-

+

+

vi

-

Solution Small Signal Analysis : circuit with the MOSFET model implicitly utilized.Voltage gain

1/gm

D

gmvsg

+

vsg

-

+

vi

-

Solution Small Signal Analysis : circuit with the MOSFET model implicitly utilized.Voltage Gain

+

vi

-

Solution Small Signal Analysis : circuit with the MOSFET model implicitly utilized.Voltage Gain

Solution Small Signal Analysis : circuit with the MOSFET model implicitly utilized.Voltage Gain

-

+

+

vi

-

Solution Small Signal Analysis circuit with the MOSFET model implicitly utilized.

Problem 6-127(e) circuit with the MOSFET model implicitly utilized.

DC Analysis 6-127(e) circuit with the MOSFET model implicitly utilized.

Small Signal Model circuit with the MOSFET model implicitly utilized.

Small Signal Model circuit with the MOSFET model implicitly utilized.

Small Signal Model circuit with the MOSFET model implicitly utilized.

Rin

Rout

Problem6-127(f) circuit with the MOSFET model implicitly utilized.Replacing BJT with MOSFET

Small Signal Model circuit with the MOSFET model implicitly utilized.

Small Signal Model circuit with the MOSFET model implicitly utilized.

Small Signal Model circuit with the MOSFET model implicitly utilized.

Rin

Rout

Problem 6-127(f) circuit with the MOSFET model implicitly utilized.

Solution P6-127(f) circuit with the MOSFET model implicitly utilized.

+

vbe2

-

+

veb1

-

+ circuit with the MOSFET model implicitly utilized.

vbe2

+

-

+

vi

veb1

-

-

Solution P6-127(f)

Problem 6-127(f) circuit with the MOSFET model implicitly utilized.with MOSFET

Solution P6-127(f) circuit with the MOSFET model implicitly utilized.

+

vgs2

-

+

vsg1

-

+ circuit with the MOSFET model implicitly utilized.

vgs2

-

+

vsg1

-

Solution P6-127(f)

ig1=0

+

vi

-

Comparison BJT/ circuit with the MOSFET model implicitly utilized.MOSFET Cct

Small Signal Model circuit with the MOSFET model implicitly utilized.

Problem 6-123 circuit with the MOSFET model implicitly utilized.

VBE=0.7 V

β =200

K’n(W/L)=2mA/V2

Vt=1V

Figure P6.123

DC Analysis circuit with the MOSFET model implicitly utilized.

Figure P6.123

V circuit with the MOSFET model implicitly utilized.BE=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 circuit with the MOSFET model implicitly utilized.

Small Signal Model circuit with the MOSFET model implicitly utilized.

Small Signal Model : circuit with the MOSFET model implicitly utilized.Voltage Gain

ig=0

+

+

vi

vbe2

-

-

Small Signal Model : circuit with the MOSFET model implicitly utilized.Input Resistance

ii

ig=0

+

vi

-

Rin