This document discusses feedback amplifiers. It outlines that feedback amplifiers have a mixer network that combines the input and feedback signals, and a sampling network that extracts a portion of the output signal for feedback. There are two main types of feedback amplifiers: positive feedback amplifiers where the feedback gain increases with increasing loop gain, and negative feedback amplifiers where the feedback gain decreases with increasing loop gain. Negative feedback amplifiers have advantages like increased stability, improved frequency response, and reduced noise and distortion.

1. Feedback Amplifier-2
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2. Outlines
1 Introduction to Feedback Amplifier
2 Mixer network
3 Sampling network
4 Types of Feedback Amplifier
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3. Introduction to Feedback Amplifier
⇒ Basic amplifier with gain A may have four types, i.e voltage, current,
transconductance and transresistance amplifier.
⇒ A = Av = Vo
Vs
, A = AI = Io
Is
, A = GM = Io
Vs
, and A = Rm = Vo
Is
⇒ Mixer network → Summer circuit.
⇒ A > 1 and β < 1
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4. Continued–
Mixer network → Across input
Series mixing
Shunt mixing
Sampling network → Across output
Voltage sampling
Current sampling
Voltage series feedback: Voltage amplifier
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5. Transconductance amplifier
Current series
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6. Current amplifier
Current shunt
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7. Transresistance amplifier
Voltage shunt
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8. Types of feedback amplifier
1 Positive feedback amplifier
Feedback Gain (Af ) =
A
1 − βA
2 Negative feedback amplifier
Feedback Gain (Af ) =
A
1 + βA
Advantage of negative feedback
(a) Increases stability
(b) Significant improvement in frequency response
(c) Device operate in linear region
(d) Bandwidth increases
(e) Noise and distortion reduces
(f) Ri and Ro can be modified according to the application.
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9. Numerical
Q If a device has open loop gain is 70 and gain with feedback is 60.
What will be the feedback factor.
Q If a device has open loop gain is 60 and gain with feedback is 80.
What will be the feedback factor and comment on the type of
feedback.
Q An amplifier without feedback has a voltage gain 50, input resistance
is 1kΩ and output resistance of 2.5kΩ. The input resistance of
current shunt negative feedback amplifier using the above amplifier
with a feedback factor of 0.2 is
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