Overview
Feedback:
Part A - Basics
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Slides taken from:
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A.R. Hambley, Electronics, © Prentice Hall,
2/e, 2000
The Concept of Feedback
Effects of feedback on Gain
Effects of feedback on non linear distortion
Effects of feedback on noise
Effects of feedback on input and output impedance
Types of feedback networks
Design of feedback amplifiers
Effect of Feedback on Bandwidth
z Transient and frequency response
z Effect of feedback on pole location
z Gain margin and phase margin
z Dominant-pole compensation
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Feedback
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Negative Feedback Pro & Cons
Consists of returning part of the output of a
system to the input
Negative Feedback: a portion of the output
signal is returned to the input in opposition to
the original input signal
Positive Feedback: the feedback signal aids
the original input signal
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Negative Feedback Effects:
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Reduces gain
Stabilizes gain
Reduces non linear distortion
Reduces certain types of noise
Controls input and output impedances
Extends bandwidth
The disadvantage of reducing the gain can be
overcome by adding few more stages of amplification
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Effects of Feedback on Gain (1)
Effects of Feedback on Gain (2)
Figure 9.1 Feedback amplifier. Note that the signals are denoted as xi, xf, xo, and so on.
The signals can be either currents or voltages.
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Problems with Positive Feedback
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Gain Stabilization (1)
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If we design the amplifier so that Aβ >> 1, then
the closed loop gain Af is approximately 1/β
Under this condition Af depends only on the
stable passive components (resistor or
capacitors) used in the feedback network,
instead of depending on the open loop gain A
which in turn depends on active device
parameters (gm) which tend to be highly
variable with operating point and temperature
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Gain Stabilization (2)
The Summing Point Constraint
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Reduction of non linear distortion (1)
Figure 9.2 Transfer characteristic of a certain nonlinear amplifier.
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Reduction of non linear distortion (2)
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Figure 9.3 Output of amplifier of Figure 9.2 for xin = sin(vt).
Notice the distortion resulting from the nonlinear transfer characteristic.
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Reduction of non linear distortion (3)
Reduction of non linear distortion (4)
Figure 9.5 Predistorted input signal.
Figure 9.4 Addition of a linear high-gain preamplifier and negative feedback
to reduce distortion.
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Reduction of non linear
distortion (5)
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Example: crossover distortion (1)
Figure 9.7 Nonlinear class-B
power amplifier.
Figure 9.5 Predistorted input signal.
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Figure 9.8 Transfer characteristic for
the amplifier of Figure 9.7.
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Example: crossover distortion (2)
Example: crossover distortion (3)
Figure 9.10 Waveforms for the circuit of Figure 9.9
with the switch in position A.
Notice the crossover distortion in the output.
Figure 9.9a Class-B power amplifier with feedback.
Figure 9.11 Waveforms for the circuit of Figure 9.9
with the switch in position B. Notice the
predistortion of the base drive voltage vB.
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Noise Reduction
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SNR for a Feedback Amplifier (1)
SNR = signal to noise ratio
Figure 9.12 Models that account for the addition of noise in amplifiers.
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Figure 9.13 Feedback amplifier with a noise source.
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SNR for a Feedback Amplifier (2)
Types of Feedback
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There are 4 basic types of feedback that have
different effects:
series – voltage
series – current
z parallel – voltage
z parallel – current
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Series-Voltage Feedback
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Series-Current Feedback
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Parallel-Voltage Feedback
Parallel-Current Feedback
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Sampling the output signal
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Units of the feedback ratio
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In complex circuits, sometimes it is not clear
whether we have current or voltage feedback
A simple test is to open or short the load
If “opening” the load the feedback signal
vanishes we have current feedback
If shorting the load the feedback vanishes we
have voltage feedback
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The units of β are the inverse of the units of
the amplifier gain
For series-voltage feedback A=Av and
β is unit less
For series-current feedback A=Gm and
β is in Ω
For parallel-voltage feedback A=Rm and
β is in Siemens
For parallel-current feedback A=Ai and
β is unit less
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Effects of various types of feedback on gain
Input impedance: effect of series feedback
series-voltage:
series-current:
parallel-voltage:
parallel-current:
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Input impedance: effect of parallel feedback
Figure 9.16 Model for analysis of the effect of parallel feedback on input impedance.
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Figure 9.15 Model for analysis of the effect of series feedback on input impedance.
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Output impedance: effect of voltage feedback
Figure 9.17 Model for the analysis of output impedance with voltage feedback.
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Output impedance: effect of current feedback
Figure 9.18 Model for the analysis of output impedance with current feedback.
Summary: Effects of feedback
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Analysis of feedback amplifiers
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Examples of feedback amplifiers (1)
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Step 1
Identify negative feedback
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Step 2
Identify the type of feedback (current feedback
vs. voltage feedback)
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Step 3
Determine the feedback ratio β = xf / xo
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Examples of feedback amplifiers (2)
Examples of feedback amplifiers (3)
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Examples of feedback amplifiers (4)
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Design of feedback amplifiers (1)
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Step 1
Decide what type of feedback is required and determine the value
of the feedback ratio
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Step 2
Select the appropriate circuit for the feedback network
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Step 3
select the appropriate valued for the components in the feedback
network
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Step 4
Analyze the circuit to verify that all approximation were legitimate.
Signal sources have nonzero internal resistance. The feedback
network has non ideal input and output impedances. Consequently
it loads the amplifier output and inserts impedance into the input
circuit
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Design of Feedback Amplifiers
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Series feedback
try to select small resistance values, so that the network does
not insert significant resistance into the input circuit
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Parallel feedback
try to select large resistance values so that the feedback
network does not tend to short out the input terminals
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Voltage feedback
try to select large feedback resistance to do not load the
amplifier
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Current feedback
try to select small feedback resistances because the input of the
feedback network is in series with the load
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