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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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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

‘T’ Model

## Single Stage MOS Amplifier

Amplifiers Configurations

### 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 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

### 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

### Small Signal Model

Small Signal Model

Small Signal Model

Rin

Rout

### Small Signal Model

Small Signal Model

Rin

Rout

+

vbe2

-

+

veb1

-

+

vbe2

+

-

+

vi

veb1

-

-

+

vgs2

-

+

vsg1

-

+

vgs2

-

+

vsg1

-

ig1=0

+

vi

-

VBE=0.7 V

β =200

K’n(W/L)=2mA/V2

Vt=1V

Figure P6.123

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

ig=0

+

+

vi

vbe2

-

-

ii

ig=0

+

vi

-

Rin