Frequency Response ELZ 303 - Elektronik II · Microelectronic Circuits - Fifth Edition Sedra/Smith The Low Frequency Response In many cases the zeros are at such low fr. as to be

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Frequency Response

ELZ 303 - Elektronik II

Dr. Mehmet Siraç Özerdem

Elektrik Elektronik Müh. Bölümü

Dicle Üniversitesi

Microelectronic Circuits – Fourth Edition

Adel S. Sedra, Kenneth C. Smith, 1998 Oxford University Press

Copyright 2004 by Oxford University Press, Inc.

s-Domain Analysis

Microelectronic Circuits - Fifth Edition Sedra/Smith

Impedance 1/sC

Impedance sL

Voltage transfer function T(s)=Vo(s)/Vi(s)

Dr. MS Özerdem

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Copyright 2004 by Oxford University Press, Inc.Microelectronic Circuits - Fifth Edition Sedra/Smith

T(s), in general

a and b are real numbers

m ≤ n m: numerator

n: denominator (order of network)

Microelectronic Circuits - Fifth

Edition Sedra/Smith


First Order Function

General form

Low pass - STC network

High pass - STC network

Dr. MS Özerdem

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Example (Bode Plot)


Example (Bode Plot)

Dr. MS Özerdem

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The Amplifier Transfer Function

dc amplifier ac amplifier or

Capacitively coupled amp

BW = wH – wL Since wL « wH BW ≈ wH

Gain-Bandwidth product

GB = AmwH


The Gain Function A(s)

General form

Dr. MS Özerdem

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The Low Frequency Response

In many cases the zeros are at such low fr. as to be of

little importance in the determining wL

Usually one of the poles – say, wP1- has much higher fr

than other poles. It follows that w close to midband.

TF of a first order high pass network


If a dominant low frequency pole does not exist, an

approximate formula can be derived for wL

Example

at w=wL

Dr. MS Özerdem

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Example


The High Frequency Response

In many cases the zeros are at such high fr. as to be of

little significance in determining wH

Usually one of the poles – say, wP1- has much lower fr

than other poles. It follows that w close to midband.

TF of a first order low pass network

Dr. MS Özerdem

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If a dominant high frequency pole does not exist, the

upper 3_dB fr wH can be determined from a plot of

An approximate formula for wH.


Example

The dominant pole is wH ≈ 104 rad/s

The better estimate

The exact value of wH can be determined from the

given transfer function as 9537 rad/s

Dr. MS Özerdem

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Using Short-Circuit and Open-Circuit Time Constants

for the Approximate Determination of wL and wH

In many cases, it is not a simple matter to determine the poles

and zeros. In such cases, approximate values of wL and wH can

be determined using the following method.

b1 can be obtained by considersing the other capacitors to zero

(open circuit )


Example

Common source FET amp.

R=100kohm

Rin=420kohm

Cgs=Cgd=1pF

gm=4mA/V

RL=3.33kohm

a) Midband voltage gain

b) fH=?

Dr. MS Özerdem

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Example


Next we outline the use of short-circuit time constant to

determine the lower 3_dB frequency, wL.

Dr. MS Özerdem

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Low Frequency Response of

the Common-Source and Common-Emitter Amplifiers

Analysis of the Common-Source Amplifier


Analysis of the Common-Source AmplifierDr. MS Özerdem

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Example

R=100kohm

RG1=1.4Mohm

RG2=0.6Mohm

RD=5kohm

RS=3.5kohm

RL=10kohm

ro=∞

Vp=-2V

IDSS=8mA

Am=?

CC1, CC2, CS=?


Low Frequency Response of


Analysis of the Common-Emitter Amplifier

Dr. MS Özerdem

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Equivalent circuit

Copyright 2004 by Oxford University Press, Inc. 24



The determination of wL

1. Find RC1 seen by CC1

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2. Find RE” seen by CE




3. Find RC2 seen by CC2

Dr. MS Özerdem

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Example


R=4kohm

R1=8kohm

R2=4kohm

RC=6kohm

RE=3.3kohm

RL=4kohm

ro=100kohm

rX=50ohm

VCC=12V

Find the midband gain Am=?

IE=1mA → βo=100

Copyright 2004 by Oxford University Press, Inc. 28Microelectronic Circuits - Fifth Edition Sedra/Smith

Example (Solution)

Set the capacitors short circuit

Dr. MS Özerdem

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Example


R=4kohm

R1=8kohm

R2=4kohm

RC=6kohm

RE=3.3kohm

RL=4kohm

ro=100kohm

rX=50ohm

VCC=12V

Find

RC1=? RC2=? RE’=? fL=?

IE=1mA → βo=100

CC1=CC2=1µF CE=10µF


High Frequency Response of


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High Frequency Response of



Let’s use Miller’s Theorem

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First order low pass filter

Dr. MS Özerdem

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Example

ro=∞

Vp=-2V

IDSS=8mA

wH = ?

Cgs= Cgd = 1pF

CC1 ,CC2 ,and Cs will

be short circuit for

estimate wH

R=100kohm

RG1=1.4Mohm

RG2=0.6Mohm

RD=5kohm

RS=3.5kohm

RL=10kohm

VDD=20V


Example (solution)Ri

Dr. MS Özerdem

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Verification

Node G

Node D


Verification Dr. MS Özerdem

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Example


R=4kohm

R1=8kohm

R2=4kohm

RC=6kohm

RE=3.3kohm

RL=4kohm

ro=100kohm

rX=50ohm

VCC=12V

AM= -22.5 V/V

a) Use Miller theorem and determine

dominant high fr pole.

b) Obtain transfer function and verify

that the dominant pole is colse to value

obtained in a)

IE=1mA → βo=100

Cπ=13.9 pF Cμ=2 pF

Copyright 2004 by Oxford University Press, Inc.Microelectronic Circuits - Fifth Edition Sedra/SmithMicroelectronic Circuits - Fifth Edition Sedra/Smith

Example (solution)Dr. MS Özerdem

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Analysis of the Common-Base Amplifier


Analysis of the Common-Base AmplifierDr. MS Özerdem

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Analysis of the Common-Base Amplifier

Documents

Frequency Response ELZ 303 - Elektronik II · Microelectronic Circuits - Fifth Edition Sedra/Smith The Low Frequency Response In many cases the zeros are at such low fr. as to be