Concept of Feedback
By;
Shamima Yasmin Sejuti
Lecturer (Provisional)
Dept. of EEE
Varendra University, Rajshahi
Definition
Feedback Amplifier is a device that is based on the principle
of feedback. The process by which some part or fraction of
output is combined with the input is known as feedback.
Types of Feedback
Positive Feedback amplifier
It is a type of an amplifier in which source signal and the feedback
signal are in the same phase. Thus, the feedback signal applied
increases the strength of the input signal.
Negative Feedback amplifier
In this type of amplifier source signal and the feedback signal are out of
phase with each other. Thus, the feedback signal applied to decrease
the strength of the input signal.
Concept of Feedback
Here, we are talking about the general condition in which
we are providing feedback to the input. It can be a positive
feedback or a negative one.
An input signal Vs is applied to the amplifier with gain A,
that produces an amplified signal, Vo.
A portion or fraction of this Vo is then fed to a feedback
network having gain β. The output of feedback network is
Vf, this signal is then given to summer or a mixer that
resultantly produces either sum or difference of the two
signal depending on their phase relationship.
Types of Negative Feedback
1. Voltage Series Feedback
2. Voltage Shunt Feedback
3. Current Series Feedback
4. Current Shunt Feedback
Voltage Series Feedback
In this type a part of output voltage fed back in series with the input
signal , resulting in overall gain reduction . It reduces the output
impedance and increase the input resistance.
Voltage Shunt feedback
In this type a part of output voltage feedback in shunt (parallel) with
an input signal . The voltage-shunt feedback provides a stabilized
overall gain and decrease both input and output resistance.
Current Series Feedback
In current series feedback a voltage is develop which is proportional to the output
current. This is called current feedback even though it is a voltage that subtracted from
the input voltage . Because of the series connection at the input and output , the input
and output resistance get increased . This type of amplifier is called transconductance
amplifier.
Current Shunt Feedback
It is called as series derived , shunt fed feedback . The shunt connection
at the input reduces the input impedance and series connection at the
output increases the output impedance.
Reduction in Frequency Distortion
For a negative-feedback amplifier having βA >>1, the gain
with feedback is Af ≈ 1/β . It follows from this that if the
feedback network is purely resistive, the gain with feedback
is not dependent on frequency even though the basic
amplifier gain is frequency dependent. Practically, the
frequency distortion arising because of varying amplifier
gain with frequency is considerably reduced in a negative-
voltage feedback amplifier circuit.
Reduction in Noise and Nonlinear
Distortion
Signal feedback tends to hold down the amount of noise signal
(such as power-supply) and nonlinear distortion. The factor
(1+βA ) reduces both input noise and resulting nonlinear
distortion for considerable improvement. However, there is a
reduction in overall gain (the price required for the improvement
in circuit performance). If additional stages are used to bring the
overall gain up to the level without feedback, the extra stage(s)
might introduce as much noise back into the system as that
reduced by the feedback amplifier. This problem can be
somewhat alleviated by readjusting the gain of the feedback
amplifier circuit to obtain higher gain while also providing
reduced noise signal.
Effect of Negative Feedback on Gain
and Bandwidth
