Lecture12 Feedback

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EE 359 Electronic Circuits

0

October 9, 20070

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Introduction

Feedback loop gain, stability


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HW and Quiz

HW #7 due Oct. 15

8.18,8.45,8.63,8.70

Midterm II Oct 29


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March 22, 20072

Introduction

Physical systems incorporate some form of

feedback

Theory of negative feedback developed by

Electrical Engineers.

Feedback theory used in modeling of

systems.

Negative or Positive Feedback


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March 22, 20073

Today

Stability

Loop Eqn


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March 22, 20074

Sampling & Mixing

Here, we look at feedback ckts

voltage-sampling series-mixing

(series-shunt) topology

F

F

A

A

x

xA

xxAxx

s

f

f

i

!!

!!

1

0

0

0


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March 22, 20075

current-sampling shunt-mixing(shunt-series) topology


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March 22, 20076

current-sampling

series-mixing

(series-series)

topology


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March 22, 20077

voltage-sampling

shunt-mixing

(shunt-shunt)topology.


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March 22, 20078

Series-shunt feedback amplifier:

Ideal ckt

Equivalent ckt.


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March 22, 20079

Measuring the output

resistance of the

voltage-samplingseries-mixing (series-

shunt) topology

feedback amplifier of

Rof| Vt/I.


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March 22, 200710

Derivation of the A circuitandF circuit for theseries-shunt feedbackamplifier.

(a) Block diagram of a

practical series-shuntfeedback amplifier.

(b) The circuit in (a) withthe feedback networkrepresented by its h

parameters.

(c) The circuit in (b) afterneglecting h21.


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March 22, 200711

Summary of the rules for finding the A circuit

and F for the voltage-sampling series-mixing


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March 22, 200712

Example 8.1

Amplifier shown,

find simplified

feedback models


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March 22, 200714

(a) idealstructure;

(b) equivalentcircuit.

Series-series feedback amplifie


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March 22, 200715

Ideal structure for the shunt-shunt feedback amplifier.


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March 22, 200716

Shunt-shunt feedback amplifier.


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March 22, 200717

Finding theAcircuit andF for

the voltage-

sampling shunt-

mixing (shunt-shunt) case.


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March 22, 2007

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Ideal structure

Shunt-series feedback amplifier.


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March 22, 2007

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Block diagram

Shunt-series feedback


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March 22, 2007

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Finding the A circuit

andF for the

current-sampling

shunt-mixing

(shunt-series) case.


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March 22, 2007

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Example 8.4


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March 22, 2007

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Example 8.4.


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Amplifier instability

March 22, 2007

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)()(1)()(

180180

180180

[F[[[

jjAjAjAf

!

999.0)()( 180180 !F jjA

A(j180) =999

(j180) = -0.001

)sin(1.0)( 180ttxi [! )sin(100)( 180ttxo [!

)sin(1.0)(180

ttxf

[!


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March 22, 2007

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Stability analysis

using Bode plot of

|A|.


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March 22, 2007

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Frequency compensation

forF = 10-2.

The response labeledAis

obtained by introducing anadditional pole at fD.

TheA response is

obtained by moving the

original low-frequency poleto fD.

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Lecture12 Feedback