Lecture (2)
Feedback Amplifiers
Definition of Feedback
❑ A small part of the output signal is fed-back to the input signal to be Added to it ( positive Feedback)
or Subtracted from it ( negative feedback).
Types of Feedback
❑ Positive Feedback:
Used to build Oscillators.
❑ Negative feedback:
Used to build Stable amplifier gain.
Advantages of Negative Feedback
1. Desensitize the gain:
Make the value of the gain less sensitive to variations in the values of circuit components, such as
might be caused by changes in temperature.
2. Reduce nonlinear distortion:
Make the output proportional to the input (in other words, make the gain constant, independent
of signal level).
3. Reduce the effect of noise:
Minimize the contribution to the output of unwanted electric signals generated, either by the
circuit components themselves, or by extraneous interference.
4. Control the input and output resistances:
Raise or lower the input and output resistances by the selection of an appropriate feedback topology.
5. Extend the bandwidth of the amplifier.
Disadvantages of Negative Feedback
❑ Reduce the amplifier Gain.
Negative Feedback Amplifiers
❑ Xs: The signal source (input of the circuit)
❑ Xi : The amplifier input
❑ Xo : The amplifier output
❑ Xf : The feedback signal
Some Properties of Negative Feedback
Gain Desensitivity:
Gain Sensitivity (S)
Example
A designer is required to achieve
a closed loop gain of 25 ± 1 % v/v
using a basic amplifier whose
gain variations is ± 10 %. What is
the nominal values of the
amplifier gain A and the
feedback factor β.
Solution:
Example
An amplifier with a nominal gain
A = 1000 V/V exhibits a gain
change of 10% as the temperature
changes from 25 o C to 75 o C. It is
required to constrain the change
to 0.1 % by applying negative
feedback. Calculate the largest
closed loop gain possible (Af) .
Solution:
Types of Amplifiers
1. Voltage Amplifier (Series-Shunt FB)
2. Current Amplifier (Shunt-Series FB)
3. Trans-conductance Amplifier (Series-Series FB)
4. Trans-Resistance Amplifier (Shunt-Shunt FB)
Analysis of the Feedback Amplifier Circuits
Steps:
Small Signal AC Model
1. BJT Model
2. MOSFET Model
Feedback Topologies
1. Series-Series Feedback:
Input= Voltage (Vs) 🡪 Voltage Source
Output = Current (Io) 🡪 Amp.out is a Current Source
2. Series-Shunt Feedback:
Input= Voltage (Vs) 🡪 Voltage Source Series Input 🡪 Voltage
Output = Voltage(Vo) 🡪 Amp.out is a Voltage Source
Shunt Input 🡪 Current
3. Shunt-Series Feedback:
Input= Current (Is) 🡪 Current Source
Output = Current (Io) 🡪 Amp.out is a Current Source Series Out 🡪 Current
4. Shunt-Shunt Feedback: Shunt Out 🡪 Voltage
Input= Current (Is) 🡪 Current Source
Output = Voltage (Vo) 🡪 Amp.out is a Voltage Source
Input and Output Resistances (Rin and Rout )
Example
Series-Shunt
Solution:
