Feedback Amplifier

1. Feedback Amplifier
Dr. Varun Kumar
Dr. Varun Kumar (IIIT Surat) 1 / 9

2. Outlines
1 Introduction to Feedback Amplifier
2 Classification of an Amplifier
Voltage amplifier
Current amplifier
Transconductance amplifier
Transresistance amplifier
3 Feedback Concept
Dr. Varun Kumar (IIIT Surat) 2 / 9

3. Introduction to Feedback Amplifier
Feedback amplifier: It is a process to modify the characteristics of
an amplifier by combining a portion of the output signal with the
external signal.
Classification of Amplifier
1 Voltage amplifier
2 Current amplifier
3 Transconductance amplifier
4 Transresistance amplifier
Dr. Varun Kumar (IIIT Surat) 3 / 9

4. Voltage Amplifier
⇒ Above figure shows the Thevenin’s equivalent circuit of a two port
network.
⇒ If the amplifier input resistance Ri is large compared to source
resistance then Vs ≈ Vi .
⇒ If the external load resistance RL is large compared to with output
resistance Ro then Vo ≈ Av Vi ≈ Av Vs
Dr. Varun Kumar (IIIT Surat) 4 / 9

5. Continued–
⇒ Above amplifier provides a voltage output proportional to the voltage
input.
⇒ Proportionality factor is independent of the magnitude of the source
and load resistance.
⇒ Such circuit is called a voltage amplifier.
Current Amplifier
An ideal current amplifier provides an output current proportional to the
signal current and proportionality factor is independent of Rs and RL.
Dr. Varun Kumar (IIIT Surat) 5 / 9

6. Continued–
⇒ An ideal current amplifier must have zero input resistance Ri and
infinite output resistance Ro.
⇒ Ri << Rs and Ro >> RL. IL ≈ Ai Ii
Transconductance Amplifier
The ideal transconductance amplifier supplies an output current which is
proportional to signal voltage.
Dr. Varun Kumar (IIIT Surat) 6 / 9

7. Continued–
⇒ This amplifier has infinite input and output resistance, i.e Ri → ∞
and Ro → ∞
Transresistance Amplifier
⇒ For practical transresistance amplifier Ri << Rs and Ro << RL.
⇒ Vo ≈ RmIi or Rm ≡ Vo
Ii
with RL = ∞
Dr. Varun Kumar (IIIT Surat) 7 / 9

8. Feedback Concept:
Representation of any single-loop feedback connection around basic
amplifier
Signal source: This block is either a signal voltage Vs in series with
resistor Rs or a signal current Is in parallel with a resistor Rs.
Dr. Varun Kumar (IIIT Surat) 8 / 9

9. Continued–
Feedback Network: This block is usually a passive two port network
which may contain resistor, capacitor, and inductors. Most often it is
simply a resistive configuration.
Sampling Network: In below figure, the output voltage is sampled
by connecting the feedback network in shunt across the output.
In another figure, the feedback network is connected in series with the
output.
Dr. Varun Kumar (IIIT Surat) 9 / 9