Hmmm interesting.
I used this R3 = 10k on the simulator and it was okey, but I dunno.

View attachment 272811

But maybe you are right.
At least I know that for the gain = positive it is negative feedback, for gain = infinite is positive feedback but for the gain = negative then it can be positive or negative feedback. But I dunno about this negative gain.

That simulator is not accurate. Though it is the 'correct' solution, it is not stable. Recommend using another spice tool like LTSpice:

To learn the most though, breadboard up a few Op-amp circuits and observe the outputs. A good simulator will give you the same answer as a real circuit if it is a good simulator.

Most of the people on this forum use LTSpice. In the future this will allow you to share results with others by using the simulator that the community tends to use the most.

 

May I ask you one more thing ?
What is this negative feedback 1/11 and positive 1/2 ?

I've not seen this in anyone analizys. Where did you get it ? This is some kind of a additional criteria, but I couldn't find any information about that.

 

May I ask you one more thing ?
What is this negative feedback 1/11 and positive 1/2 ?

I've not seen this in anyone analizys. Where did you get it ? This is some kind of a additional criteria, but I couldn't find any information about that.

Simple: The voltage divider rule:
The negative feedback is just the output multiplied by R4/(R3+R4) and the positive feedback is the output multiplied by R1/(R1+R2). In both cases it is a simple fraction of the output voltage fed back to the inputs of the op-amp. Thus 1/11 that is 1/(1+10) or 1/2 that is 1/(1+1). Lookup voltage divider rule, it will tell you why it works that way. Keep in mind when applying the rule you must short the input voltage to zero (in this case to ground) to see the contribution of the positive feedback per the superposition rule.

 

Simple: The voltage divider rule:
The negative feedback is just the output multiplied by R4/(R3+R4) and the positive feedback is the output multiplied by R1/(R1+R2). In both cases it is a simple fraction of the output voltage fed back to the inputs of the op-amp. Thus 1/11 that is 1/(1+10) or 1/2 that is 1/(1+1). Lookup voltage divider rule, it will tell you why it works that way. Keep in mind when applying the rule you must short the input voltage to zero (in this case to ground) to see the contribution of the positive feedback per the superposition rule.

I thought I couldn't use it because I was calculating the V+ and V- with a specific equations
And As I understand you grounded the Vin so it was 0V (it was 1V before). To see what is the gain in both feedbacks ?
It's like we are using Vout as an only source.

So not only I have to calculate the gain like before (for example using this "lim" method) but also I have to check the gain of both feedbacks ? Hmm it is pretty complex. Maybe that's why both feedbacks aren't even used. But It's good to know how to calculate them.

So to sum things up :

- calculate the Vout/Vin using some methods like IN+ = IN - or "lim" method or other.
- calculate the feedback gains to check if it's still positive or negative feedback "dominance".

 

I thought I couldn't use it because I was calculating the V+ and V- with a specific equations
And As I understand you grounded the Vin so it was 0V (it was 1V before). To see what is the gain in both feedbacks ?
It's like we are using Vout as an only source.

So not only I have to calculate the gain like before (for example using this "lim" method) but also I have to check the gain of both feedbacks ? Hmm it is pretty complex. Maybe that's why both feedbacks aren't even used. But It's good to know how to calculate them.

So to sum things up :

- calculate the Vout/Vin using some methods like IN+ = IN - or "lim" method or other.
- calculate the feedback gains to check if it's still positive or negative feedback "dominance".

Sure, that is one way to look at it. I recommend you learn the principles of superposition, voltage divider rule, Thevenin's theorem, and Norton's theorem. They all derive from ohms law and Kirchhoff's current law and voltage law.

 

May I ask you one more thing ?
What is this negative feedback 1/11 and positive 1/2 ?

I've not seen this in anyone analizys. Where did you get it ? This is some kind of a additional criteria, but I couldn't find any information about that.

You requested to ask "one more thing" for the nth time and received a perfectly good answer. You have not demonstrated any effort beyond internet search engines to understand the principles involved. I suggest (again) that you do so.