Negative Feedback in Operational Amplifiers: Understanding Op Amp Output, Slides of Electronics

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Op amp output

Electronics

Negative Feedback in Operational Amplifiers

Terry Sturtevant

Wilfrid Laurier University

July 29, 2013

The equation for the output of an op amp is always true as long as the output is not saturated

The equation for the output of an op amp is always true as long as the output is not saturated

V +

V −

Vo

Vo = A ( V + − V −)

For negative feedback, make the voltage at V − be some portion of Vo.

For negative feedback, make the voltage at V − be some portion of Vo.

This ratio of V − to Vo is called the feedback factor , β.

β

V +

V −

Vo



6

For instance, with just a wire from V − to Vo , the feedabck factor is 1.

For instance, with just a wire from V − to Vo , the feedabck factor is 1.

This is the case for a voltage follower.

β = 1

V +

V −

Vo

 

For feedback

For feedback V − = βVo

where

For feedback V − = βVo

where β ∈ [ 0 , 1 ]

For feedback V − = βVo

where β ∈ [ 0 , 1 ]

So the op amp equation

Vo = A ( V + − V −)

becomes

For feedback V − = βVo

where β ∈ [ 0 , 1 ]

So the op amp equation

Vo = A ( V + − V −)

becomes Vo = A ( V + − βVo )

Vo + βAVo = AV +

Vo + βAVo = AV +

Vo ( 1 + βA ) = AV +