Questions about Operational Amplifier with feedback

[Shayaan_Mustafa]

Hello experts!

I have some question regarding with Op-amp.

Q#1) We know that we use Op-amp with closed loop not with open loop. But why we use negative feedback why don't positive feedback?

Q#2) Here is diagram of the simple Op-amp with feedback. We know due to high input impedance of the Op-amp the current flows through the feedback resistor as shown in the figure below. Right?

But when output is received(i.e. Vo) across RL then some part is fed back to the input in order to control the gain. Right? But the current flows in the reverse direction as shown in the figure below.

I am just confused how one feedback provides two way current, like this,
Blue is current from output to input and Red is the current from input to output due to high input impedance
Does my thinking is wrong?

Thanks in advance.

 

[goldsmith]

Re: Operational Amplifier

Q#1) We know that we use Op-amp with closed loop not with open loop. But why we use negative feedback why don't positive feedback?

Hi Shayaan
because Positive feedback will shift poles to the unstable region thus your system will be unstable . and don't forget that your open loop gain is pretty high . with negative feedback you can decrease it ( set it in your desired range ) and of course thus BW will increase , as well !

Q#2) Here is diagram of the simple Op-amp with feedback. We know due to high input impedance of the Op-amp the current flows through the feedback resistor as shown in the figure below. Right?

Yes it is correct .

But when output is received(i.e. Vo) across RL then some part is fed back to the input in order to control the gain. Right? But the current flows in the reverse direction as shown in the figure below.

Exactly wrong ! don't forget that current will go through the RF and don't forget that inverting input can be considered as an imaginary ground ! thus the out put voltage have 180 degree phase shift !

I am just confused how one feedback provides two way current, like this,

Why do you think that we have two current ? don't forget principles of imaginary ground !
Do you know how your op amp can give you your desired gain ?

Best Wishes
Goldsmith

 

[KRIS255]

Re: Operational Amplifier

The way to look at an op-amp with negative feedback as shown in your drawings is as follows:
1. The voltage between the two input teminals is always zero.
2. Since the positive op-amp input is connected to ground (0 volts), the negative input must also be zero volts.
3. This means the current through the input resistor must be Vin/Rj.
4. Since the op-amp input resistance is very-very high, all of the current going through the input resistor must go through the feedback resistor.
5. Since one end of the feedback resistor is 0 volts (from sentence 2), the other end of the feedback resistor must be Vout = -(Vin/Rj)*Rf.
6. The gain of the op-amp is therefore G = -Rj/Rf.

Jim Christensen

 

[goldsmith]

Re: Operational Amplifier

Hi Jim
I think you made a mistake !

G = -Rj/Rf.

G=-RF/Ri
Regards
Goldsmith

 

[Shayaan_Mustafa]

Re: Operational Amplifier

Hello sir!

Thank you for reply.

I am still not getting about this,

Exactly wrong ! don't forget that current will go through the RF and don't forget that inverting input can be considered as an imaginary ground ! thus the out put voltage have 180 degree phase shift !

I know output will be 180 out of phase. But when we are using feedback and at input most current flow through Rf so at output some part of output is fed back to the input through feedback then why not current flow through this direction to?
Means one direction is when input current flows through Rf from Vin to Vo. And other should be from Vo to Vin because we are using Rf in order to feed some part of output to the input.

- - - Updated - - -

@KRIS255

Your points are already cleared to me. But I thank you.

 

[jasonc2]

Re: Operational Amplifier

This is a quick and easy read and will help answer all of the questions you have posted, I learned a lot from it myself:

https://www.allaboutcircuits.com/vol_3/chpt_8/1.html

Quick link to negative feedback section: https://www.allaboutcircuits.com/vol_3/chpt_8/4.html
Quick link to positive feedback section: https://www.allaboutcircuits.com/vol_3/chpt_8/12.html

 

[goldsmith]

Re: Operational Amplifier

Shayaan , see below , please :

Best wishes
Goldsmith

 

[Shayaan_Mustafa]

Re: Operational Amplifier

Hello sir..

That is what I am trying to say you so far. In your own diagram, look, you have draw current from blue line through Rf and with red line through Rf. This is I am asking for, how only one feedback path provides blue line and red line current at a time?

BTW: Your mentioned point in the diagram are actually cleared to my mind but I am not getting what are you trying to say from these points. !!!!!

 

[LvW]

Re: Operational Amplifier

Hello sir..

That is what I am trying to say you so far. In your own diagram, look, you have draw current from blue line through Rf and with red line through Rf. This is I am asking for, how only one feedback path provides blue line and red line current at a time?

Yes, the red current is false - because of the virtual ground assumption for the n-input..

 

[godfreyl]

Re: Operational Amplifier

I am just confused how one feedback provides two way current, like this,
View attachment 78248 Blue is current from output to input and Red is the current from input to output due to high input impedance

Blue and red arrows should point the same way. It is the same current. When input voltage is positive, then output voltage is negative. So current flows only one way - from input to output.

See the example below. If input = 1V, then output = -3V and current = 1mA.

 

[goldsmith]

Re: Operational Amplifier

Hi dear LvW and dear godfreyl
I think you misunderstood my meaning , my mean is when input is positive , out put is negative and the current through RF is that blue shape . and when input is negative the out put is positive and the current is negative . Red and blue are instead of different inputs . aren't you gentlemans agree with me ?
Best Regards
Goldsmith

 

[LvW]

Re: Operational Amplifier

Blue and red arrows should point the same way. It is the same current. When input voltage is positive, then output voltage is negative. So current flows only one way - from input to output.
See the example below. If input = 1V, then output = -3V and current = 1mA.

Yes, of course that's correct.
As another view:
Connect a series combination two resistors (R1=1k, R2=3k from left to right) between two voltage sources (V1=+1V, V2=-3V).
Then calculate the current through both resistors and the potential against ground at the node between both resistors.
Suddenly, the "virtual ground" becomes a "real" ground.

- - - Updated - - -

Hi dear LvW and dear godfreyl
I think you misunderstood my meaning , my mean is when input is positive , out put is negative and the current through RF is that blue shape . and when input is negative the out put is positive and the current is negative . Red and blue are instead of different inputs . aren't you gentlemans agree with me ?
Best Regards
Goldsmith

Yes - agreed, but this was not clear from the drawing. Why do you use two different voltages with inverse polarity in one single drawing?
This is really confusing - and, more than that, we gain absolutely nothing from using the negative input voltage.
All currents reverse in direction. So what?

 

[goldsmith]

Re: Operational Amplifier

Yes - agreed, but this was not clear from the drawing. Why do you use two different voltages with inverse polarity in one single drawing?
This is really confusing - and, more than that, we gain absolutely nothing from using the negative input voltage.
All currents reverse in direction. So what?

Hi dear LvW
Oh yes pardon me , my words were not clear . and of course i thought there isn't any problem if i draw both of them in a picture . but i didn't want to create confusion . you are absolutely right and i should drew them in two pictures . hence sorry because of created confusion .
Sincerely Yours
Goldsmith

 

[LvW]

Re: Operational Amplifier

i should drew them in two pictures . hence sorry because of created confusion .

Why two drawings - for two input voltages of opposite polarity?
I think this is an example which demonstrates that too much (redundant) information can cause confusion.

 

[goldsmith]

Re: Operational Amplifier

at too much (redundant) information can cause confusion.

Hi dear LvW
Why i drew that ! let me tell the reason ! the author of thread have asked about currents through the resistors . i thought it is a good way if i demonstrate him that instead of each half cycle ( or each polarity ) there will be just one current . instead of input polarity it's direction will have change . isn't that a good manner to describe , in your opinion ? ( of course your idea is pretty important for me and all of the times , correct . )
Best Regards
Sincerely
Goldsmith

 

[Shayaan_Mustafa]

Re: Operational Amplifier

@LuW
As you are saying to Goldsmith that his drawing is confusing to users. So, let me say it is not correct. He is helping me on a thread where he teach me with this single drawings have two current directions. And I think it is a good way to visualize different angles of circuit in one drawing. Isn't it.

So by @godfreyl and LvW discussion I concluded that there is only one direction of the current through Rf i.e. from input to output (If) and if I look this from output to input then it will be as (-If). Right?