Feedback Amplifiers - DPG Polytechnic
Transcript of Feedback Amplifiers - DPG Polytechnic
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Feedback Amplifiers
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Outline
• Introduction
• The general feedback structure
• Some properties of negative feedback
• The four basic feedback topologies
• The series-shunt feedback amplifier
• The series-series feedback amplifier
• The shunt-shunt and shunt-series feedbackamplifier
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•The stability problem•Stability study using bode plot•Frequency compensation
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Introduction
• It’s impossible to think of electroniccircuits without some forms of feedback.
• Negative feedback
Desensitize the gain
Reduce nonlinear distortion
Reduce the effect of noise
Control the input and output impedance
Extend the bandwidth of the amplifier
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• The basic idea of negative feedback is totrade off gain for other desirableproperties.
• Positive feedback will cause the amplifieroscillation.
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The General Feedback Structure
This is a signal-flow diagram, and the quantities
x represent either voltage or current signals.
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The General Feedback Equation
• Closed loop and open loop
• Closed loop gain
• Feedback factor β
• Loop gain Aβ
• Amount of feedback (1+ Aβ)
A
A
x
xA
s
of
1
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Some Properties of Negative
Feedback
• Gain desensitivity
• Bandwidth extension
• Noise reduction
• Reduction in nonlinear distortion
A
dA
AA
dA
f
f
1
1
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The Four Basic Feedback Topologies
• Voltage amplifier---series-shunt feedback
voltage mixing and voltage sampling
• Current amplifier---shunt-series feedback
Current mixing and current sampling
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• Transconducatnce amplifier---series-
series feedback
Voltage mixing and current sampling
• Transresistance amplifier---shunt-
shunt feedback
Current mixing and voltage sampling
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The Series-Shunt Feedback
Topologies
voltage-mixing voltage-sampling (series–
shunt) topology
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The Amplifier with Series-Shunt
Feedback
voltage-mixing voltage-sampling (series–
shunt) topology
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The Shunt-Series Feedback Topologies
current-mixing current-sampling (shunt–
series) topology
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The Amplifier with Shunt-Series
Feedback
current-mixing current-sampling (shunt–
series) topology
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The Series-Series Feedback
Topologies
voltage-mixing current-sampling (series–
series) topology
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The Amplifier with Series-Series
Feedback
voltage-mixing current-sampling (series–
series) topology
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The Shunt-Shunt Feedback
Topologies
current-mixing voltage-sampling (shunt–
shunt) topology
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The OP Amplifier with Shunt-Shunt
Feedback
current-mixing voltage-sampling (shunt–
shunt) topology
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The Series-Shunt Feedback Amplifier
• The ideal situation
• The practical situation
• summary
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The Ideal Situation
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A unilateral open-loop amplifier (A
circuit).
An ideal voltage mixing voltage
sampling feedback network (β circuit).
Assumption that the source and load
resistance have been included inside
the A circuit.
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The Ideal Situation
Equivalent circuit.
Rif and Rof denote the input and output resistance
with feedback.
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Input and Output Resistance with
Feedback
• Input resistance
In this case, the negative feedbackincreases the input resistance by afactor equal to the amount of feedback.
)1( ARR iif
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• Output resistance
In this case, the negative feedbackreduces the output resistance by a factorequal to the amount of feedback.
A
RR o
of
1
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The Practical Situation
Block diagram of a practical series–shunt feedback
amplifier.
Feedback network is not ideal and load the basic amplifier
thus affect the values of gain, input resistance and output
resistance.
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The Practical Situation
The circuit in (a) with the feedback network
represented by its h parameters.
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The Practical Situation
The circuit in (b) with h21 neglected.
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The Practical Situation
• The load effect of the feedback network on
the basic amplifier is represented by the
components h11 and h22.
• The loading effect is found by looking into
the appropriate port of the feedback network
while the port is open-circuit or short-circuit
so as to destroy the feedback.
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• If the connection is a shunt one, short-
circuit the port.
• If the connection is a series one, open-
circuit the port.
• Determine the β.
02
112
1
IV
Vh
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Summary
• Ri and Ro are the input and output resistances,
respectively, of the A circuit.
• Rif and Rof are the input and output resistances,
respectively, of the feedback amplifier, including
Rs and RL.
• The actual input and output resistances exclude Rs
and RL.
Loutof
sinif
RRR
RRR
//
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Example of Series-Shunt Feedback
Amplifier
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Example of Series-Shunt Feedback
Amplifier
• Op amplifier connected in noninverting
configuration with the open-loop gain μ, Rid
and ro
• Find expression for A, β, the closed-loop
gain Vo/Vi , the input resistance Rin and the
output resistance Rout
• Find numerical values
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Example of Series-Shunt Feedback
Amplifier
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Example of Series-Shunt Feedback
Amplifier
21
1
RR
R
V
V
s
f
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The Series-Series Feedback
Amplifier
• The ideal situation
• The practical situation
• summary
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The Ideal Situation
Trans conductance gain i
o
V
IA
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The Ideal Situation
o
f
I
VTranresistance feedback factor
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Input and Output Resistance with
Feedback
• Input resistance
In this case, the negative feedbackincreases the input resistance by afactor equal to the amount of feedback.
)1( ARR iif
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• Output resistance
In this case, the negative feedbackincreases the output resistance by afactor equal to the amount of feedback.
)1( ARR oof
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The Practical Situation
Block diagram of a practical series–series
feedback amplifier.
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• Feedback network is not ideal and load
the basic amplifier thus affect the
values of gain, input resistance and
output resistance.
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The Practical Situation
The circuit of (a) with the feedback network
represented by its z parameters.
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The Practical Situation
A redrawing of the circuit in (b) with z21
neglected.
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The Practical Situation
• The load effect of the feedback network
on the basic amplifier is represented by
the components Z11 and Z22.
• Z11 is the impedance looking into port 1 of
the feedback network with port 2 open-
circuited.
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• Z22 is the impedance looking into port
2 of the feedback network with port 1
open-circuited.
• Determine the β.
02
112
1
II
Vz
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Summary
• Ri and Ro are the input and output
resistances, respectively, of the A circuit.
• Rif and Rof are the input and output
resistances, respectively, of the feedback
amplifier, including Rs and RL.
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• The actual input and output resistances
exclude Rs and RL.
Loutof
sinif
RRR
RRR
'
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Example of Series-Series Feedback
Amplifier
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Example of Series-Series Feedback
Amplifier
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Example of Series-Series Feedback
Amplifier
1
12
2E
EFE
E
o
fR
RRR
R
I
V
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Example of Series-Series Feedback
Amplifier
)33
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rRrgrR
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RrRRRR
ofomoout
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The Shunt-Shunt and Shunt-Series
Feedback Amplifiers
• Study by yourselves
• Important notes:
Closed-loop gain
Feedback factor
Load effect
Summary
example
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The Stability Problem
• Closed-loop transfer function is similar to the one of the middle band gain.
• The condition for negative feedback to oscillate
1)()()( jjAjL
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• Any right-half-plane poles results ininstability.
Amplifier with a single-pole isunconditionally stable.
Amplifier with two-pole is alsounconditionally stable.
Amplifier with more than two poles hasthe possibility to be unstable.
• Stability study using bode plot
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The Definitions of the Gain and
Phase margins
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Gain margin represents the amount by
which the loop gain can be increased while
stability is maintained.
Unstable and oscillatory
Stable and non-oscillatory
Only when the phase margin exceed 45º or
gain margin exceed 6dB, can the amplifier
be stable.
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Stability analysis using Bode plot of |A|
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Stability Analysis Using Bode Plot of |A|
• Gain margin and phase margin
• The horizontal line of inverse of feedback
factor in dB.
• A rule of thumb:
The closed-loop amplifier will be stable if
the 20log(1/β) line intersects the 20log|A|
curve at a point on the –20dB/decade
segment.
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• The general rule states:
At the intersection of 20log[1/ | β (jω)| ]
and 20log |A(jω)| the difference of slopes
should not exceed 20dB/decade.
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Frequency Compensation
• The purpose is to modifying the open-loop
transfer function of an amplifier having
three or more poles so that the closed-loop
amplifier is stable for any desired value of
closed-loop gain.
• Theory of frequency compensation is the
enlarge the –20dB/decade line.
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• Implementation
Capacitance Cc added
Miller compensation and pole splitting
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Frequency Compensation
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• Two cascaded gain stages of a
multistage amplifier.
• Equivalent circuit for the interface
between the two stages in (a).
• Same circuit as in (b) but with a
compensating capacitor CC added.
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• Frequency compensation for = 102.
The response labeled A is obtained by
introducing an additional pole at fD. The
A response is obtained by moving the
original low-frequency pole to f D.
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Frequency Compensation
A gain stage in a multistage amplifier with
a compensating capacitor connected in the
feedback path
An equivalent circuit.