Negative Feedback in Operational Amplifiers

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Negative Feedback in Operational Amplifiers

• CEC

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Feedback
Af A/(1 - Aß) in oscillators
Af A/(1 Aß) in amplifiers
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Feedback

• Comparators, Zero Crossing Detectors operate
  without feedback Open Loop leads to OPAMP
  saturation.
• Amplifiers, Oscillators have feedback Closed
  Loop controls gain.
• Negative Feedback in Amplifiers.
• Positive Feedback in Oscillators, Schmitt
  Triggers.

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Feedback Factor

• Ratio of the feedback voltage to output voltage
  known as feedback factor or feedback ratio.
• Gain decreases with negative feedback, increases
  with positive feedback.

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Feedback
Amplifier
Amplifier
Feedback Network
Feedback Network
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Positive Feedback

• Also called Regenerative Feedback.
• Closed loop with feedback, Open loop if no
  feedback.
• Feedback energy (voltage or currents) inphase
  with the input signal and thus aids it.
• Increased amplifier gain.
• Increases distortion, noise and instability.
• Used in oscillator circuits.

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Negative Feedback

• Also called Degenerative feedback.
• Closed loop when an op amp has feedback.
• Negative Feedback return of a portion of the
  output signal to the input signal
  (out-of-phase). 
• Reduces effective input.
• Will effectively attenuate the final output.
• Used in amplifiers. 

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Negative Feedback

• Feedback energy (voltage or current) is outof
  phase with the input signal and thus opposes it.
• Reduces amplifier gain, also reducedistortion,
  noise and instability.
• Increases bandwidth, improves input and output
  impedances close to ideal.

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Parameter Changes
Noise
Noise Reduced
Gain depends on feedback fraction, Gain Stability
improved
Distortion Reduced
Bandwidth Increased
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Advantages of Negative Feedback

• Stabilizes Amplifier Gain.
• Reduces Non-linear Distortion.
• Increases Circuit Stability.
• Increases Input Impedance/Resistance.
• Decreases Output Impedance/Resistance.
• Reduces Noise Level.
• Improves Frequency Response Bandwidth.
• More Linear Operations.

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Comparison
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Feedback Topologies
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Feedback in Amplifiers

• Current Series.
• Voltage Series.
• Current Shunt.
• Voltage Shunt.

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Amplifier Feedback Topologies
Voltage Series Feedback
Voltage Shunt Feedback
Current Shunt Feedback
Current Series Feedback
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Input Resistance
Input impedance of an op-amp circuit with
negative feedback different from the open loop
input resistance.
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Output Resistance
Output resistance with feedback different from
output resistance without feedback
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Comparison of Feedback Topologies
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Voltage Series Feedback
Feedback Network
Noninverting Configuration
ß R2/(R1 R2)
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Voltage Series Feedback

• Portion of the o/p voltage applied to the input
  voltage in series through the feedback circuit.
• o/p impedance will be reduced and i/p impedance
  will be enlarged.
• RofRo/(1Aß) and Rif Ri(1Aß).
• Voltage series feedback in common collector or
  emitter follower amplifier.

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Voltage Shunt Feedback
Vcc
- Vcc
Inverting Configuration
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Voltage Shunt Feedback

• Portion of the o/p voltage applied to the input
  in parallel with, through the feedback circuit.
• Both o/p  impedance i/p impedance will be
  decreased.
• RofRo/(1Aß) and Rif Ri/(1Aß).
• Voltage shunt feedback in transresistanceamplifie
  rs.

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References

• Your prescribed reference textbooks.

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Thank You