The document discusses the benefits of negative feedback (NFB) in amplifiers, highlighting its roles in improving amplifier stability, input and output impedance, bandwidth, reducing distortion, and minimizing noise. By employing NFB, amplifiers achieve better stability and performance across varying frequencies, ultimately enhancing output fidelity. Overall, NFB is essential for improving the quality and reliability of electronic amplification systems.

1. NFB SYSTEMS AND BENEFITS
FIG 1 FIG 2
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2. BJT AND NFB CONCEPT
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3. NFB PRACTICAL BENEFIT (1) –IMPROVED
AMPLIFIER STABILITY
 The generalized closed loop feedback equation derived from a non inverting
amplifier is
 Transistor amplifiers are totally unreliable because the GAIN of all
transistors varies with temperature which will change component
values, leading to instability of amplifier frequency responses.
 Equation above shows that, by introducing NEGATIVE FEEDBACK, the
parameter A0β (called LOOP GAIN) will investigate the circuit stability.
Negative Feedback REDUCES the overall GAIN of an amplifier.
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4. AMPLIFIER STABILITY
 The great significance is that, the overall gain of the
amplifier (closed loop gain Af ) now depends almost
exclusively on β.
 Since β depends on the ratio of the feedback network
with FIXED COMPONENTS, R1, and R2, (fig 2) the
amplifiers output stability is maintained.
 For example, if an amplifiers output changes for any
reason, because of the FIXED FEEDBACK COMPONENTS
introduced, the output change affects the input in such a
way as to subtract from the input signals.
 This will counteract the output change hence, producing
a STABLE amplifier circuit response.
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5. NFB BENEFIT (2) –INPUT
IMPEDANCE ANALYSIS
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6. Cont…………..
 Let Vd = small differential voltage between the
two
inputs.
 The input voltage can be expressed as
Vin = Vd + Vf
 Let β = Ri/(Ri + Rf). Then β Vout = Vf. Thus,
Vin = Vd + BVout
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7. Cont………..
But Vout = AolVd (i.e. the open loop gain
times the differential voltage). So:
Vin = Vd +β AolVd = (1 + Aolβ )Vd
Substituting IinZin for Vd to get
Vin = (1 + Aolβ) IinZin
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8. Cont…………
 Where Zin is the open-loop input impedance of the op-
amp (i.e. impedance without feedback connections).
 The overall input impedance of a closed-loop (Zin(NI))
for non-inverting amplifier configuration is:
Zin(NI) = Vin/Iin = (1 + Aolβ) Zin
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9. Benefit ONE:
Effect on Input Impedance
Equation 1 shows that, the input
impedance of an amplifier with
negative feedback is much
greater than (INCREASED) the
internal input impedance of the op-
amp itself (i.e. without feedback
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10. NFB BENEFIT (3) –OUTPUT
IMPEDANCE ANALYSIS
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11. Cont…………
Apply Kirchhoff’s law to the output circuit to
get:
 Vout = AolVd – ZoutIout
 The differential input is Vd = Vin – Vf.
Assume AolVd >> ZoutIout. Then,
 Vout = Aol(Vin – Vf)
 The differential input is Vd = Vin – Vf.
 Assume AolVd >> ZoutIout. Then,
 Vout = Aol(Vin – Vf)
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12. Cont……….
 Let β = B. Substitute BVout­for Vf to get
 Vout = Aol(Vin – BVout) = AolVin – AolBVout
 AolVin = Vout + AolBVout = (1 + AolB)Vout
 The output impedance of the non-inverting amplifier
is Zout(NI) = Vout/Iout.
 Substitute IoutZout(NI) for Vout:
 AolVin = (1 + AolB) IoutZout(NI)
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13. Cont………
OR….
 AolVin/Iout = (1 + AolB) Zout(NI)
Without feedback, AolVin = Vout. Thus,
 Zout = (1 + AolB) Zout(NI)
With Feedback
Zout(NI) = Zout / (1 + AolB)
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14. Benefit Three:
Effect on Output impedance
Thus the output impedance of an
amplifier with negative
feedback is much less
(DECREASED) than the internal
output impedance, Zout, of the op-
amp itself( i.e. without feedback).
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15. NFB BENEFIT (4) –EFFECT ON
BANDWIDTH
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16. EFFECT ON BANDWIDTH
 Typically Op Amps are used for comparatively low
frequency circuits. But for improved performance,
negative feedback could be employed.
 Negative Feedback is used to INCREASE the
BANDWIDTH of an amplifier (speed it up) at the cost of
lowering the amplifier gain
 Thus, the stabilization of gain increases the effective
bandwidth. It flattens and extends the frequency response
of an amplifier.
 Negative Feedback is a major contribution to extending the
useful frequency range of amplifiers.
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17. What is Frequency Response
 Some components of an amplifier have frequency limitations
and so respond differently as the frequency changes.
 Since the response of components varies with the frequency,
the components of an amplifier must be selected to amplify a
certain range or band of frequencies.
 Hence, how well (i.e. linearity) an amplifier will perform
without attenuation is described by frequency response.
 The frequency response of an amplifier refers to the frequency
range that the amplifier is designed to amplify. A High
Fidelity amplifier must exhibit a flat response from 20Hz –
20kHz
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18. Cont……….
 The frequency range in which the amplifier will
operate with negligible effects from capacitors,
inductors, and device internal capacitance is called
Mid-Frequency Range or Mid Band Frequency.
 Bandwidth refers to the ‘band’ of frequencies for
which the amplifier provides adequate amplification.
 Bandwidth is normally measured at the 3dB point (s)
of the frequency response.
 3dB is the frequency at which the output signal is
cutoff to 70.7% of the input signal value.
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19. Plotting Frequency Response Curve
 A frequency-response curve is a graphical representation of
the relationship between amplifier gain and operating
frequency.
 In such graphs, Gain is plotted against Frequency. It is
common that very large values may be encountered for both
gain and frequency. To ensure that wide range of values
are plotted on a single graph both the frequency and gain
axes of the graph may use logarithmic scales.
 The (vertical) y-axis uses linear divisions or logarithmic units
in (decibels dB).
 The (horizontal) x-axis is a frequency scale which has a large
range of quantities and so does not increase in a linear
manner. For this reason it uses the logarithmic scale.
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20. Graphical Representation of
Frequency Response Curve
The curve shows how gain, measured in deciBels (dB)
varies with frequency.
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21. Frequency response curve
 At frequencies above the mid band(high frequency
range) and below the mid band (low frequency range),
the device’s amplification is less than adequate.
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22. Frequencies above and below
Mid-band
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23. Frequency Response….cont
 All transistors show less gain as the frequency
increases. WHY ?
 The high frequency limit for a transistor is related
to the junction capacitances called MILLER
EFFECT named after JOHN MILTON MILLER.
 Miller Effect is the effect of a larger capacitance
introduced between the transistor’s base (in BJTs)
or gate (in FETs) and its collector or drain, which
LIMITS a transistor’s GAIN at HIGH
FREQUENCIES.
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24. MILLER EFFECT
 Note that Vcc is +
 The collector-base capacit
 ance CCB and emitter-base
 capacitance CEB decrease
 the gain of a common emitter
 circuit at higher frequencies.
 The Miller effect makes Ccb
look β times larger at the base
 of a CE amplifier.
 The reactance of a capacitor decreases as the frequency
increases. This decreasing reactance loads the amplifier,
hence dropping the gain.
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25. TRANSISTORS GAIN AND
FREQUENCY VARIATIONS
Why all transistors show LESS GAIN as FREQUENCY
DECREASES or INCREASES.
 At frequencies above and below the midrange,
capacitance and any inductance will affect the gain of the
amplifier.
 At low frequencies the coupling and bypass capacitors
lower the gain.
 At high frequencies stray capacitances associated with the
active device lower the gain.
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26. BENEFIT FIVE: NFB EFFECT ON
AMPLIFIER DISTORTION
Due to the parameters of a signal, the process of amplification
introduces some DISTORTIONS. Reactive components in
amplifiers will introduce frequency and phase distortions, while
Amplitude distortion is mostly caused by poor DC biasing.
 Distortion in amplifiers is ANY CHANGE in the BASIC NATURE of
an input signal waveform.
 The use of negative feedback can discriminate against sources of
distortion within an amplifier.
 Thus, Negative Feedback REDUCES DISTORTION generated in the
stages of an amplifier.
 Introducing Negative feedback to reduce distortion will
IMPROVE the FIDELITY of an amplifier by reproducing the EXACT
NATURE of the input signal at the output stage.
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27. NOISE IN AMPLIFIERS
External Generated
 Occurs across the whole bandwidth or frequency
spectrum of an amplifier
 May come from entirely natural sources such ‘static’
noise in the form of hissing and cracking radiated from
space
 Atmospheric noise generated by lightning discharges
in thunderclouds.
 Transmissions broadcasting at similar frequencies to
the required signal.
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28. NOISE IN AMPLIFIERS
 All signals are contaminated with noise as unwanted sound.
 Internally Generated
 Resistors, Transistors, especially bipolar types, will generate
noise from a variety of causes including Thermal noise caused
by heat.
 Shot noise’, which is caused by charge carriers (electrons and
holes) randomly diffusing across the semiconductor junctions.
 Flicker Noise which is found in all semiconductor devices
under the application of a current bias.
 Avalanche Noise- caused by Zener or avalanche
breakdown in a pn junction
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29. BENEFIT SIX: NFB EFFECT ON
NOISE
 Noise at frequencies above and below the
required bandwidth of the amplifier can be
REDUCED by Negative Feedback using high
and low pass filters.
 Negative feedback can play a part in
improving the signal to noise ratio within
the bandwidth of an amplifier.
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30. WAAAAAAOOOOOOOOOO
END
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