2 05 feedback circuit - utcluj.ro · 2/20 The general structure of the feedback circuit xs –...
Transcript of 2 05 feedback circuit - utcluj.ro · 2/20 The general structure of the feedback circuit xs –...
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�The feedback – technique use to influence the behavior and
properties of a system by the output quantity of the system
� the effect produced by a certain cause influences the consequent
action of that cause
� fed back to the input a fraction of the output quantity
Feedback circuits
Two types of feedback:
• negative feedback (degenerative) NF. The signal fed back from
the output reduces the effect of the input signal; stabilizing effect
• positive feedback (regenerative) RP. The signal fed back from the
output intensifies the effect of the input signal; leads to instability
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The general structure of the feedback circuit
xs – source signal, fed from an external signal source;
xo – output signal, fed to the load as well as to the feedback network;
xr – feedback signal, provided by the feedback network
xi – input signal to the basic amplifier, provided by the summing
element.
Signal flow
diagram
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The general structure of the feedback circuit
• transmittance of the basic amplifier :
• transmittance of the feedback network:
• transmittance of the feedback circuit:
i
o
x
xa =
o
r
x
xr =
s
o
x
xA =
gain of the basic amplifier; open loop gain
gain of the feedback amplifier
feedback factor; transmittance of the reverse path
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Equations of the ideal feedbackIdeal feedback:
• each block is unilateral (one way transmittance)
• the r circuit does not load the a circuit; that is, connecting the r
circuit does not change the value of a.
• assume that the source and the load resistance have been included
inside the a circuit
Negative
feedback
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Negative feedback
rsivvv −=
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Fundamental
equation of the NF
ar – loop gain
1+ar amount of feedback
ar > 0;
1+ar > 1
� If ar >> 1 (eg. for op amp )∞→a
The gain of the feedback amplifier is almost entirely determined
by the feedback network : accurate, predictable and stable gain.
Negative
feedback
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Exemplification
a=1000, r=0.009
ar =?
1+ar =?
A=?
111,10,009
1
r
1A ≈==
Negative feedback: - reduces the gain
- improves some amplifier properties (stabilizes
the gain, reduce nonlinear distortion, improves the input and output
impedances, extend the bandwidth).
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Positive feedback
rsixxx +=
ar
aA
−=
1
rsivvv +=
ar > 0; 0 < 1-ar < 1
Amplifier in the
linear domain
aA >
But the amplifier
performances get
worse
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� Special case of PF:
∞→==− Aarar ;1;0)1(
soAxx = xo finite only if xs=0
It is used for sinusoidal oscillator with PF (signal generator)
� 0 >1-ar; ar>1
• still have RP, but the linear theory can not be used anymore for
circuit analysis;
• specific analyzing method will be used
• multivibrator circuits, bistable circuits
Positive feedback
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The feedback topologies
• input and output signals: voltage and/or currents:
results four feedback topologies.
• the loop gain is dimensionless
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PS
PS
Illustration
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Analysis of the feedback amplifier
� Ideal feedback:
� there are no restrictions on the basic amplifier
� the other components of the circuit are made ideal
� the feedback network – a controlled source
� the signal source is assumed to be ideal (it is assumed that
the source resistance is included inside the basic amplifier)
� the load resistance does not load the output of the amplifier
(it is assumed that the load resistance is included inside the
basic amplifier)
� Real feedback:
� the feedback network loads the input and output of the basic
amplifier
� the circuit with real feedback ideal feedback
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Ideal feedback, series-parallel topology
Structure of the
ideal feedback
amplifier
Equivalent circuit
of the amplifier
A=?
Rir=?
Ror=?
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Rir
• increases the input resistance by a
factor equal to the amount of feedback.
• improves the input resistance
(amplifier with voltage as input)
Rir=?
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Ror
• reduces the output resistance by a
factor equal to the amount of feedback.
• improves the output resistance
(amplifier with voltage as output)
Output in short-circuit , vo=0
iopeno,
sco,
openo,
or
avv
i
vR
=
=
ar1
avv s
openo,+
=
o
s
o
isco,
R
av
R
avi ==
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Loads due to:
• feedback network
• source resistance
• load resistance
Real feedback, series-parallel topology
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Approximation of the real feedback by the ideal feedback
series-parallel topology
orL
Lor
orRR
RRR
′−
′=
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Illustration
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a’=195487;
r=0,091