Question about an ideal opamp analysis

hello.. i have a doubt in ideal opamp analysis.. i have attached the word document for better establishment of my question.. please see it...
my question is "why Vn approaches Vp when gain is infinity and not the vice versa ?" The book doesn't state it clearly ... i have taken it from OPamp desigh & analog ICs by franco... please answer my question.. thanks in advance...

Re: ideal opamp analysis

electroboy, you should know that this rule (Vp=Vn) holds only in case of feedback.
The background is that a very small voltage (approaching zero) across the input is sufficient to cause an output voltage - stiil within the linear operating range of the opamp (if its gain approaches infinity). This equlibrium (stabilization within the linear range) happens automatically due to the negative feedback principle.

Re: ideal opamp analysis

LvW said:

electroboy, you should know that this rule (Vp=Vn) holds only in case of feedback.
The background is that a very small voltage (approaching zero) across the input is sufficient to cause an output voltage - stiil within the linear operating range of the opamp (if its gain approaches infinity). This equlibrium (stabilization within the linear range) happens automatically due to the negative feedback principle.

Click to expand...

Yes sir .. i have understood that.. but according to the book, it tells Vn only approaches Vp and not Vp approaches Vn (in the last line) .. please say me why.. thank you very much for spending ur valuable time with me...

Re: ideal opamp analysis

electroboy said:

Yes sir .. i have understood that.. but according to the book, it tells Vn only approaches Vp and not Vp approaches Vn (in the last line) .. please say me why.. thank you very much for spending ur valuable time with me...

Click to expand...

Note, that you are welcome with your questions.
Here is my answer: Since Vn (inverting input) is connected to the feedback path the voltage at Vn is self-regulated such that it follows the voltage that is the only cause for an output voltage - and that is Vin=Vp. This holds for the non-inverting gain application. In case of an inverter we have Vp=0 and the voltage Vn is automatically adjusted to a value (very close to Vp=0) such that it keeps the opamp in its linear region.

Thus, in both cases Vp is fixed and Vn approaches this fixed value.

Re: ideal opamp analysis

As long as the opamp is in any negative feedback configuration, it tries its best (though it may not always succeed) to adjust its output such that the 2 inputs are at the same voltage (ie virtual short).
Digest the statement above and try to apply it to the voltage follower, inverting and non-inverting amplifiers. After that, consider when the opamp may fail to achieve the virtual short scenario.

---------- Post added at 17:09 ---------- Previous post was at 17:07 ----------

As long as the opamp is in any negative feedback configuration, it tries its best (though it may not always succeed) to adjust its output such that the 2 inputs are at the same voltage (ie virtual short).
Digest the statement above and try to apply it to the voltage follower, inverting and non-inverting amplifiers. After that, consider when the opamp may fail to achieve the virtual short scenario.

Re: ideal opamp analysis

thank you .. greatly helpful...