Voltage Series Negative Feedback in Operational Amplifier Circuits

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Voltage Series Negative Feedback

• CEC

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Contents

• Negative Feedback.
• Noninverting Amplifier.
• Gain with Feedback.
• Bandwidth with Feedback.
• Noise and Distortion with Feedback.
• Input and Output Resistances with Feedback.
• Total Output Offset Voltage with Feedback.
• Voltage Follower.

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Comparison of Feedback Topologies
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Negative Feedback Amplifier
Vf
Input applied to the noninverting terminal
Vf
Feedback Factor ß R2/(R1 R2) Vf/VOUT
Drop across R2 fed back in phase opposition to
the input.
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Negative Feedback

• Closed loop configuration through feedback from
  output to input.
• OPAMP amplifies the difference of two input
  voltages ie amplifies Vid Vin Vf.
• Feedback voltage opposes the input voltage ie
  180o out of phase.
• Feedback is negative.

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Noninverting Amplifier

Vf
Voltage Series Feedback
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Noninverting Amplifier

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Amplifier Gain

• Gain without feedback (Open loop voltage gain) A
  , hence vo Avd A(v1 v2)
• where differential input vd v1 v2 vin
  - vf
• Gain with feedback (Closed loop voltage gain) Af
  
• Gain of the feedback circuit ß
• Now, v1 vin and v2 vf
• Substitute and solve for Af

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Amplifier Gain with Feedback

• Af since Aß
  gtgt1.
• Since open loop gain A is very large, AR1 gtgt (R1
  Rf) and hence Af 1
• Gain with feedback controlled by controlling
  feedback, entirely dependent on feedback factor.
• Gain stability not dependent on other parameters,
  hence improved.
• All external components to be less than 1MO.

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Bandwidth with Feedback

• Bandwidth - Range of frequencies for which the
  gain remains constant.
• Gain Bandwidth product constant, equal to unity
  gain bandwidth UGBAfo.
• For a single break frequency
• OPAMP, A.fo Af.ff where
• Af gain with feedback.
• ff , but Af

Open loop Gain
Frequency (Hz)
fb
Break Frequency
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Bandwidth with Feedback

• Substitute for Af , we get
• ff fo(1 Aß)
• Bandwidth increases with feedback.

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Noise with Feedback

• Noise fed back out of phase with input noise and
  added along with signal.
• Noise cancellation with negative feedback.
• Negative feedback reduces amplifier gain, hence
  the impact of noise.
• Noise with feedback Nf

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Distortion with Feedback

• Distortion reduced with negative feedback.
• Negative feedback reduces amplifier gain, hence
  also the distortion present in the signal.
• Distortion with feedback Df .

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Input Resistance with Feedback

• Negative feedback increases input resistance.

Feedback Resistor Rf moved to input side.
Rf moved to output
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Input Resistance with Feedback
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Output Resistance with Feedback
Resistance determined by looking back into the
feedback amplifier from the output terminals.
Rof
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Output Resistance with Feedback

• Ratio of output voltage to output current.
• Rof since output
  resistance is very low and a majority of current
  flows into the output terminals of the OPAMP.
• Apply KVL to the output loop, io
• vid v1 v2 0 vf - -
  ßvo.
• Substitute for vid, find io and Rof,

Output Resistance decreases with feedback
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Total Output Offset Voltage with Feedback

• Input offset voltages and currents amplified to
  produce output offset parameters.
• Gain reduces with feedback.
• Total output offset voltage also decreases with
  feedback by a factor

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Voltage Follower

• For voltage follower Af ß 1 and 1 A A,
  hence
• Input resistance Rif ARi.
• Output resistance Rof .
• Bandwidth ff Afo, and
• Total output offset voltage VooT

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References

• OPAMPS and Linear Integrated Circuits, Ramakanth
  A Gayakwad, 4th Edition, Pearson Education.
• Your prescribed text books.

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Thank You