Feedback in amplifier

Physics
BSc III Sem V
Shri Shivaji Science College, Amravati
Dr. Vaishali V. Deshmukh

FEEDBACK IN AMPLIFIERS
Oscillators and Multivibrators
2
1

Concept of Feedback
What is feedback in an amplifier
Topologies of Feedback
Voltage and current feedback
Types of Feedback
Positive and negative feedback
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What is an amplifier
Basic Amplifier

What is an Amplifier?
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▰ A device which accept an input signal and produces an
output signal proportional to the input, is called an amplifier.
VS : is the signal voltage,
RS : is the internal resistance of the source,
V1 : is the actual input voltage to the amplifier,
I1 : is the input current to the amplifier,
V2 : is the output voltage across the load RL,
I2 : is the output current flowing through the load RL.

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✓The important function of an amplifier is the amplification.
✓Circuit that increases the amplitude of the weak signals.
✓The important parameters of an amplifier are input impedance, output
impedance, current gain and voltage gain.
✓A good design of an amplifier circuit must possess high input impedance,
low output impedance and high current gain.
✓Constitute an essential part of radio, television and communication devices.
✓In discrete circuits, bipolar JFET, FET is used as amplifying element.
✓We find the use of amplifiers in medical devices, scientific equipment,
automation, military tools, communication devices, and even in household
equipment.

Limitations of basic amplifier
▰ The instability of a.c. gain is due to the change in
supply, change in h-parameters, age of the device.
▰ The input and output impedance are not according
to the requirements of the basic amplifier.
▰ Distortion is large.
▰ Noise is more.
▰ Bandwidth is not sufficiently large.
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FEEDBACK AMPLIFIER
Feedback is the process of taking a
proportion of an amplifier´s output
signal and feeding it back into the
input. Feedback can be arranged to
increase or decrease the input
signal.
An amplifier that has a feedback
connection established between the
output to the input is known as the
feedback amplifier.

Positive
The positive feedback can be defined
as when the feedback current
otherwise voltage is applied for
increasing the i/p voltage, then it is
named as positive feedback. Direct
feedback is another name of this
positive feedback. Because positive
feedback generates unnecessary
distortion; it is not often used in
amplifiers. But, it amplifies the original
signal power and can be used in
oscillator circuits.
Types of feedback
Negative
The negative feedback can be defined as
if the feedback current otherwise voltage
can be applied for reducing the amplifier
i/p, then it is called as negative feedback.
Inverse feedback is another name of this
negative feedback. This kind of feedback
is regularly used in amplifier circuits.
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Advantages of negative feedback
▰ Stability of gain is improved
▰ Reduction in distortion
▰ Reduction in noise
▰ Increase in input impedance
▰ Decrease in output impedance
▰ Increase in the range of uniform application
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Though the gain of negative feedback amplifier is reduced, there are many
advantages of negative feedback such as,
It is because of these advantages negative feedback is frequently
employed in amplifiers.

Comparison between Negative & Positive feedback
▰ When the feedback voltage or current is in
phase with the input signal and thus aids
it, it is called positive feedback.
▰ Positive feedback increases the gain of
the amplifier.
▰ Noise in the output signal increases
▰ Distortion increases
▰ Bandwidth decreases
▰ It is used in oscillators
▰ When the feedback voltage or current is
out of phase with the input signal and thus
opposes it, it is called positive feedback.
▰ Negative feedback reduces the gain of the
amplifier.
▰ Noise in the output signal decreases
▰ Distortion decreases
▰ It is used in amplifiers
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Positive Feedback Negative Feedback

Principle of Feedback Amplifier
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A feedback amplifier generally consists of two parts. They are the amplifier and the feedback circuit. A feedback
network is a linear two-port network that contains resistors, inductors, capacitors. Its function is to fed back some
portion of output to the input.
From the above figure, the gain of the amplifier is represented as A. the gain of the amplifier is the ratio of output
voltage Vo to the input voltage Vi. the feedback network extracts a voltage Vf = β Vo from the output Vo of the amplifier.

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This voltage is added for positive feedback and subtracted for negative
feedback, from the signal voltage Vs. Now,
Vi = Vs + Vf = Vs + βVo
Vi = Vs – Vf = Vs − βVo
The quantity β = Vf/Vo is called as feedback ratio or feedback fraction.
Let us consider the case of negative feedback. The output Vo must be equal
to the input voltage (Vs - βVo) multiplied by the gain A of the amplifier.
Hence,
(Vs−βVo) A=Vo
Or
AVs−AβVo = Vo
Or
AVs = Vo (1+Aβ)
Therefore,
Vo/Vs=A/ (1+Aβ)

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Let Af be the overall gain (gain with the feedback) of the amplifier. This is defined as the ratio
of output voltage Vo to the applied signal voltage Vs, i.e.
Af = Output voltage /Input signal voltage = Vo/Vs
So, from the above two equations, we can understand that, The equation of gain of the feedback
amplifier, with negative feedback is given by
Af = A/ (1+Aβ)
The equation of gain of the feedback amplifier, with positive feedback is given by
Af = A/ (1−Aβ)
Case1: when β is positive i.e. when feedback voltage is in phase with input voltage( +Ve FB)
then |1-Aβ | < 1, Af >A. Therefore, positive feedback increases the amplifier gain.
Case2: when β is negative i.e. when feedback voltage is out of phase with input voltage( -Ve FB)
then |1-Aβ | >1, Af < A.Therefore, negative feedback reduces the amplifier gain.
The term Aβ is called loop gain.

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Voltage-Series Feedback
Negative Voltage Feedback: In this method, the voltage feedback
to the input of amplifier is proportional to the output voltage.
In the voltage series feedback circuit, a fraction of the
output voltage is applied in series with the input voltage
through the feedback circuit. This is also known as shunt-
driven series-fed feedback, i.e., a parallel-series circuit.
The figure shows the block diagram of voltage series
feedback, by which it is evident that the feedback circuit
is placed in shunt with the output but in series with the
input. As the feedback circuit is connected in shunt with
the output and the input as well, both the output
impedance and the input impedance are decreased.

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Voltage-Shunt Feedback
Negative Voltage Feedback: In this method, the voltage feedback
to the input of amplifier is proportional to the output voltage.
In the voltage shunt feedback circuit, a fraction of the
output voltage is applied in parallel with the input
voltage through the feedback network. This is also
known as shunt-driven shunt-fed feedback i.e., a
parallel-parallel proto type. The figure shows the block
diagram of voltage shunt feedback, by which it is
evident that the feedback circuit is placed in shunt with
the output and also with the input. As the feedback
circuit is connected in shunt with the output and the
input as well, both the output impedance and the input
impedance are decreased.

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Current-Series Feedback
Negative Current Feedback: In this method, the voltage feedback
to the input of amplifier is proportional to the output current.
In the current series feedback circuit, a fraction of the
through the feedback circuit. This is also known as series-
driven series-fed feedback i.e., a series-series circuit. The
figure shows the block diagram of current series feedback,
by which it is evident that the feedback circuit is placed in
series with the output and also with the input. As the
feedback circuit is connected in series with the output and
the input as well, both the output impedance and the input
impedance are increased.

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Current-Shunt Feedback
Negative Current Feedback: In this method, the voltage feedback
to the input of amplifier is proportional to the output current.
In the current shunt feedback circuit, a fraction of the
through the feedback circuit. This is also known as series
driven shunt-fed feedback i.e., a series-parallel circuit.
The figure shows the block diagram of current shunt
feedback, by which it is evident that the feedback circuit
is placed in series with the output but in parallel with the
input. As the feedback circuit is connected in series with
the output, the output impedance is increased and due to
the parallel connection with the input, the input
impedance is decreased.

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Characteristics
Types of Feedback
Voltage-Series Voltage-Shunt Current-Series Current-Shunt
Voltage Gain Decreases Decreases Decreases Decreases
Bandwidth Increases Increases Increases Increases
Input resistance Increases Decreases Increases Decreases
Output resistance
Decreases Decreases Increases Increases
Harmonic
distortion
Decreases Decreases Decreases Decreases
Noise Decreases Decreases Decreases Decreases

Reference Books
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1. Principle of Electronics by V.K. Mehata
2. Basic Electronics (Solid State) by B.L. Theraja
3. Handbook of Electronics by Gupta Kumar

“Once you stop learning,
you start dying
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Albert Einstein

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THANKS!
Any questions?
You can find me at
vaishalideshmukh27@gmail.com

Feedback in amplifier

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