Small Signal Model MOS Field-Effect Transistors (MOSFETs)

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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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Presentation Transcript
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.
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.
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 Models

‘T’ Model

### Single Stage MOS Amplifier

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 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’
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
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
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) 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 Gate Circuit (CG)

Current Follower

Quiz No 4

27-03-07

• Draw/Write the Following:
SOLUTION : DC Analysis

IE

Check for Active Mode

IB

Solution P6-127(f)

+

vbe2

-

+

veb1

-

+

vbe2

+

-

+

vi

veb1

-

-

Solution P6-127(f)
Solution P6-127(f)

+

vgs2

-

+

vsg1

-

+

vgs2

-

+

vsg1

-

Solution P6-127(f)

ig1=0

+

vi

-

Problem 6-123

VBE=0.7 V

β =200

K’n(W/L)=2mA/V2

Vt=1V

Figure P6.123

DC Analysis

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