Feedback confusion

[Buddy40]

Hello to all members,

I am a student of electronics and new to this forum.
My question is related to opamps.
I will try to attach the circuit (I`ve seen elsewhere) - it has two feedback circuits and my question is
* What is the purpose of such a circuit and
* What is the gain (if it should work)?

Thank you
Buddy

 

[ericgibbs]

hi,
Your pdf file will not open, Invalid File' message

 

[Buddy40]

hi,
Your pdf file will not open, Invalid File' message

Thank you,
I will try it again.

 

[kubeek]

The circuit configuration is called howland current source, quite hard to find when you don´t know what it is. I rememberd it took me a few weeks to get to the bone of this circuit back in 2006
Use google or look at this **broken link removed**

 

[JimB]

That circuit is a "Howland Current Source".

Have a look here for a detailed explanation:
https://www.electro-tech-online.com/custompdfs/2013/06/snoa474a.pdf

JimB

 

[Buddy40]

Kubeek and JimB,

thank you for your quick reply and the attached documents.
In particular, the second one (from Jim) is very nice.
I am surprised that it is a current source - but not the opamp output, right?
I was told that at the output of the opamp there is a voltage and we have an amplifier.
Is this false?

 

[kubeek]

The opamp output sources the voltage and the resistive divider provides and monitors the output current. The negaitve input of the opamp doesn´t source this current in any way, it only measures the voltage at that point.

 

[MikeMl]

If you are using the opamps output pin as a voltage output (not as the Howland) , then here is a way of analizing it.

First, assume R3 is infinite. Then it degenerates into a conventional non-inverting amplifier, where for a finite R4, the gain is 1+R2/R1.

Now start making R3 smaller. Because R3 provides positive feedback, the overall gain of stage increases. There is a limit to how small R3 can become without making the positive feedback greater than the negative feedback via R1/R2.

The place where things go wierd is when R3/R4 = R2/R1.

I'll let you do the equations...

btw, there is a "homework" forum for questions like this...

 

[MrAl]

Hi,

An expression for the output voltage is this:

Vout=V1*R3*(R1+R2)/(R1*R3-R2*R4)

But as you can see there are going to be constraints on the resistor values (as Mike also pointed out). For example, the denominator immediately tells us that R1*R3 can not be equal to R2*R4. You can look for other problems yourself if you would like to.

 

[Buddy40]

Thank you to all answers.
Now I know about the circuit.
Thank you.