# In a real-world op-amp, how does it make sure Vn and Vp differs and don't equal each other?

I was confused about op-amps and asked this question: In an op-amp, why does Vn approach Vp until the difference is delta V / A? Why doesn't Vn just equal Vp? and received really helpful answers.

I now know why in a real-world op-amp, Vn has to differ from Vp; Vn can't just equal Vp or else the output voltage will equal 0. However, how does a real-world op-amp ensure that Vp and Vn differ by a small amount? How does it make sure that Vn doesn't increase to equal Vp? How does it ensure that the difference between Vp and Vn is Vp - (Vp - Vn) / A instead of 0?

• Given that a real amplifier has finite (but large) gain, what would the output voltage be, when the input voltage is exactly 0 V? Oct 30, 2016 at 19:26
• @ThePhoton If the input voltage is exactly 0, then output voltage is 0. That's why Vn doesn't increase until it hits Vp. However, in the real-world (and perhaps this is a hardware question), HOW does it actually ensure that Vn doesn't reach Vp but instead stops at some threshold (so that output voltage ISN'T 0)? Oct 30, 2016 at 19:33
• physically, the input voltage causes the output voltage, not the other way around. We just often calculate backwards because it makes the math simpler. Oct 31, 2016 at 16:20