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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. 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
slide1
Small Signal Model

MOS Field-Effect Transistors (MOSFETs)

1

quiz no 3 de 27 ce
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).
slide3
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
MOSFET Analysis

iD = iS, iG = 0

slide6
Large-signal equivalent-circuit model of an n-channel MOSFET : Operating in the saturation region.
slide7
Large-signal equivalent-circuit model of an p-channel MOSFET : Operating in the saturation region.
slide10
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
Small Signal Models

‘T’ Model

common source amplifier cs
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
slide23
Small-signal analysis performed directly on the amplifier circuit with the MOSFET model implicitly utilized.
common source amplifier cs summary
Common Source Amplifier (CS) Summary
  • Input Resistance is infinite (Ri=∞)
  • Output Resistance = RD
  • Voltage Gain is substantial
slide25
Common-source amplifier

with a resistance RSin the source lead

the common source amplifier with a source resistance
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 r s
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
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
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.
slide43
(a) A common-drain or source-follower amplifier :output resistance Rout of the source follower.
slide44
(a) A common-drain or source-follower amplifier. : Small-signal analysis performed directly on the circuit.
slide47
Common Gate Circuit (CG)

Current Follower

quiz no 4
Quiz No 4

27-03-07

  • Draw/Write the Following:
slide55
SOLUTION : DC Analysis

IE

Check for Active Mode

IB

solution p6 127 f
Solution P6-127(f)

+

vbe2

-

+

veb1

-

solution p6 127 f1
+

vbe2

+

-

+

vi

veb1

-

-

Solution P6-127(f)
solution p6 127 f2
Solution P6-127(f)

+

vgs2

-

+

vsg1

-

solution p6 127 f3
+

vgs2

-

+

vsg1

-

Solution P6-127(f)

ig1=0

+

vi

-

problem 6 123
Problem 6-123

VBE=0.7 V

β =200

K’n(W/L)=2mA/V2

Vt=1V

Figure P6.123

dc analysis
DC Analysis

Figure P6.123

slide96
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

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