# How opamp work with Feed back?

Discussion in 'Homework Help' started by EDFG, May 14, 2016.

1. ### ronsimpsonWell-Known MemberMost Helpful Member

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Look at R1 and R2. They make a voltage divider.
Think about if there was no op amp:
If you have +5V at the top of R1,R2.
You need 0V at the center.
What voltage must be at the bottom to get oV at the center?

Here you can see the current flow from +5V through R1, through R2, op amp output, -15V supply.

2. ### specWell-Known MemberMost Helpful Member

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Hy EDFG,

Welcome to ETO,

Care to tell us were you are and put it in 'Location' on your user page so that it displays in your user window at the left of your posts.

Here is my description of the circuit operation.

(1) First some basics about silicon transistors and operational amplifiers (opamps)
(1.1) The band gap of silicon transistors can be taken as 0.7V This is the voltage necessary, between the base and emitter, to make current flow from the collector to the emitter.
(1.2) An opamp cannot abide its two input voltages being different, so it will do all it can to make its inputs equal. The only way it can do this is by adjusting the voltage on its output. It has no other way. It will do this very fast too.

(3) Circuit function and calculation of opamp output voltage.
(3.1) The +ve input of the opamp is connected to 0V.
(3.2) This means that, if it can, the opamp will make its -ve input exactly 0V too.
(3.3) From this it follows that the current flowing from 5V through the resistor will be, 5V/1K = 5mA
(3.4) In order to keep its -ve input at exactly 0V, the opamp must remove exactly 5mA away from its -ve input.
(3.5) The only way it can do this is by altering the voltage on the emitter of the transistor so that its collector current is exactly 5mA to balance the 5mA flowing through the resistor.
(3.6) The base of the transistor is connected to 0V so the opamp must make the emitter 0.7V negative with respect to 0V so that the transistor collector current is 5mA

So the opamp will maintain its output at -0.7V, and that's all there is to it.

Last edited: May 15, 2016
3. ### DerStrom8Super ModeratorMost Helpful Member

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Spec,

EDFG was Ritesh (i.e. "koolguy") in disguise. He was banned, just like his previous aliases.

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5. ### cowboybobWell-Known MemberMost Helpful Member

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Thanks, Matt. I was beginning to wonder after EDFG's 3rd post and the nonsense meter pegged at the 6th...

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6. ### specWell-Known MemberMost Helpful Member

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Thanks Matt

chuck