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Slide1

ENEE 303 3rd DiscussionSlide2

ContentsMOSFET small signal equivalent circuit

BJT small signal equivalent circuitSlide3

From signal point of view,

MOSFET behaves as voltage-controlled current source.

Accepts

between gate and source

Provides current at drainInput resistance is highOutput resistance is high

Small-signal

models for the MOSFET:

(a) neglecting the dependence of iD on vDS insaturation (the channel-lengthmodulation effect) and (b) including the effect of channel length modulation

MOSFET: Small-Signal Equivalent Circuit

ENEE 303 Fall 2017

Y

p

- Model

T

- Model

Transconductance

Output Resistance

Same formula for PMOS except use |VOV |, |Vt|, |l|

MOSFET: Small Signal Equivalent Circuit Models

ENEE 303 Fall 2017Slide5

MOSFET: Small-Signal Equivalent Circuit

ENEE

303

Fall

2017

Summary

DC bias

AC signal

The small signal approximation:

DC bias:

Set the signal to zero to obtain the quiescent point.

AC signal: Set the DC bias voltages and currents and analyze the response of the circuit to the signal. Use the small-signal equivalent circuit.Slide6

Given:

V

t

= 1.5 V,

k

n = 0.25 mA/V2 , VA = 50 V

Find: small-signal voltage gain, input resistance, largest allowable input

signal(remain in

saturation region)Example : MOSFET Amplifier

ENEE 303 Fall

2017Slide7

Given:

V

t

= 1.5 V,

kn = 0.25 mA/V2 , VA = 50 V

Find: small-signal voltage gain, input resistance, largest allowable input signal

ENEE 303 Fall 2017

Example : MOSFET AmplifierSlide8

Given:

V

t

= 1.5 V,

k

n

= 0.25 mA/V2 , VA = 50 V

Find: small-signal voltage gain, input resistance, largest allowable input signal

ENEE 303 Fall 2017

Example : MOSFET AmplifierSlide9

Given:

V

t

= 1.5 V,

k

n

= 0.25 mA/V2 , VA

= 50 V

Find: small-signal voltage gain, input resistance, largest allowable input signalThe largest allowable input signal is constrained by,

ENEE 303 Fall 2017

Example :

MOSFET AmplifierSlide10

ENEE

303

Fall

2017

Small Signal Operation and Models: BJT

Define the

BJT

transconductance

as,

The BJT

transconductance

is the slope of the input characteristic

Slide11

ENEE

303

Fall

2017

Small Signal Operation and Models: BJT

The total base current,

The signal current at the base is then,

The small signal input resistance

between the base and emitter, looking into the base, is defined as,

Slide12

From signal point of view,

BJT behaves as voltage-controlled current source.

Accepts

between base and emitter

Provides current at the collectorInput resistance is high but finiteOutput resistance is high

Small-signal

models for the

BJT: (a) neglecting the dependence of iC on vCE in saturation (the Early effect) and (b) including the Early effect

BJT: Hybrid- Equivalent Circuit

ENEE 303 Fall 2017

Slide13

BJT: Hybrid-

Equivalent Circuit

ENEE

303

Fall

2017

Summary

DC bias

AC signal

The small signal approximation:DC bias: Set the signal to zero to obtain the quiescent point.AC signal:

Set the DC bias voltages and currents and analyze the response of the circuit to the signal. Use the small-signal equivalent circuit.

Slide14

BJT: Hybrid-

Equivalent Circuit

ENEE

303

Fall

2017Slide15

BJT: Hybrid-

Equivalent Circuit

ENEE

303

Fall

2017Slide16

The Three Transistor Amplifier Configurations

ENEE

303

Fall

2017Slide17

The input resistance is,

The output voltage is,

The output resistance is,

The overall voltage gain is,

The Common-Source (CS) Amplifier Configuration

ENEE

303

Fall

2017Slide18

The CS Amplifier with Source Resistance

ENEE

303

Fall

2017

Input Resistance

Output Resistance

Open-Circuit Voltage Gain

Amplifier and Overall Voltage Gain

reduces the amplifier gain

By: cheryl-pisano
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Type: Public

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