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EE 359 Electronic Circuits
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October 9, 20070
FeedbackFeedback
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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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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Today
Stability
Loop Eqn
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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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current-sampling shunt-mixing(shunt-series) topology
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current-sampling
series-mixing
(series-series)
topology
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voltage-sampling
shunt-mixing
(shunt-shunt)topology.
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Series-shunt feedback amplifier:
Ideal ckt
Equivalent ckt.
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Measuring the output
resistance of the
voltage-samplingseries-mixing (series-
shunt) topology
feedback amplifier of
Rof| Vt/I.
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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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Summary of the rules for finding the A circuit
and F for the voltage-sampling series-mixing
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Example 8.1
Amplifier shown,
find simplified
feedback models
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(a) idealstructure;
(b) equivalentcircuit.
Series-series feedback amplifie
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Ideal structure for the shunt-shunt feedback amplifier.
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Shunt-shunt feedback amplifier.
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Finding theAcircuit andF for
the voltage-
sampling shunt-
mixing (shunt-shunt) case.
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Ideal structure
Shunt-series feedback amplifier.
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Block diagram
Shunt-series feedback
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Finding the A circuit
andF for the
current-sampling
shunt-mixing
(shunt-series) case.
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Example 8.4
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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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Stability analysis
using Bode plot of
|A|.
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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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