This document discusses negative feedback amplifiers. It defines feedback amplifiers as those with a feedback path from the output to the input. Negative feedback decreases the overall gain but improves performance and stability. The key aspects covered are: - Negative feedback improves properties like gain stability, linearity and bandwidth while reducing sensitivity to variations. - There are four basic feedback topologies depending on whether voltage or current is mixed or sampled in the feedback path. - The loop gain, which quantifies the feedback, can be determined by breaking the feedback loop and applying a test signal. - Negative feedback provides benefits like desensitized gain, reduced nonlinear distortion and noise, and control of input/output impedances by trading

1. Determining Loop Gain
S MARIA ANTONY
Assistant Professor/ ECE Dept
KIT- Kalaignarkarunanidhi Institute of Technology

2. Feedback Amplifier
If an amplifier has feedback path exists from output to
input then that amplifier is termed as Feedback
amplifier.
Feed back is the parameter which quantifies the amount
of feedback given in a feedback amplifier.

3. TYPES OF FEEDBACK
If feedback in the amplifier decreases the overall gain,
then the feedback is termed as negative feedback
If feedback in the amplifier increases the overall gain,
then the feedback is termed as positive feedback

4. NEGATIVE FEEDBACK AMPLIFIER
It is an electronic amplifier that subtracts a fraction of its output
from its input, so that negative feedback opposes the original
signal.
The applied negative feedback improves performance (gain
stability, linearity, frequency response, step response) and reduces
sensitivity to parameter variations due to manufacturing or
environment.
An idealized negative feedback amplifier is a system of three
elements
An amplifier with gain AOL
A feedback network β, and
A summing circuit

5. The General Feedback Structure
This is a signal-flow diagram, and the quantities x represent
either voltage or current signals.

6. Introduction
• It’s impossible to think of electronic circuits 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
• The basic idea of negative feedback is to trade off gain for other desirable
properties.
• Positive feedback will cause the amplifier oscillation.

7. The General Feedback Equation
• Closed loop and open loop
• Closed loop gain
• Feedback factor β
• Loop gain Aβ
• Amount of feedback (1+ Aβ)
βA
A
x
x
A
s
o
f
+
=≡
1

8. Some Properties of Negative Feedback
• Gain desensitivity
• Bandwidth extension
• Noise reduction
• Reduction in nonlinear distortion
A
dA
AA
dA
f
f
β+
=
1
1

9. 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
• Transconducatnce amplifier---series-series feedback
Voltage mixing and current sampling
• Transresistance amplifier---shunt-shunt feedback
Current mixing and voltage sampling

10. The Series-Shunt Feedback Topologies
voltage-mixing voltage-sampling (series–shunt) topology

11. The Amplifier with Series-Shunt Feedback
voltage-mixing voltage-sampling (series–shunt) topology

12. The Shunt-Series Feedback Topologies
current-mixing current-sampling (shunt–series) topology

13. The Series-Series Feedback Topologies
voltage-mixing current-sampling (series–series) topology

14. The Shunt-Shunt Feedback Topologies
current-mixing voltage-sampling (shunt–shunt) topology

15. the loop gain can be found with the following steps:
• Break the feedback loop anywhere
• Zero out the input signal xs
• Apply a test signal to the input of the feedback circuit
• Solve for the resulting signal xo at the output
If xo is a voltage signal, xtst is a voltage and measure the open-circuit voltage
If xo is a current signal, xtst is a current and measure the short-circuit current
• The negative sign comes from the fact that we apply
negative feedback

16. A
β
Σxs=0
xf
xi
xo
xtst
A
x
x
AxAxAxx
xx
xx
tst
o
tstfio
fi
tstf
β
β
β
=−=
−=−==
−=
=
gainloop
0

17. Negative Feedback Properties
Negative feedback takes a sample of the output signal and applies it to
the input to get several desirable properties. In amplifiers, negative
feedback can be applied to get the following properties
–Desensitized gain – gain less sensitive to circuit component variations
–Reduce nonlinear distortion – output proportional to input (constant
gain independent of signal level)
–Reduce effect of noise
–Control input and output impedances – by applying appropriate
feedback topologies
–Extend bandwidth of amplifier
These properties can be achieved by trading off gain