# Understanding Negative feedback loop circuit with an Ideal Op-Amp [duplicate]

The following is given as an example of a negative feedback loop in my textbook, note that the op amp is assumed to be ideal.

However I do not seem to get the logic behind this acting as a negative feedback loop, suppose if $$\V_{in}\$$ is $$\0.01 \$$ after passing through the op amp it should be amplified to $$\+6\ \mathrm V\$$.

Now $$\V_{out}\$$ is $$\+6\ \mathrm V\$$ hence the inverting input will also become $$\+6\ \mathrm V\$$ thus the difference between the two inputs becomes $$\-5.99\ \mathrm V\$$ thus $$\V_{out}= -6\ \mathrm V\$$ and this process shall repeat over and over again, would the circuit then just cycle between being $$\+6\ \mathrm V\$$ and $$\-6\ \mathrm V\$$ until the power supply is disconnected?

How is this considered a negative feedback, am I missing something crucial?

• Tips: It's $ on EE.SE for inline MathJAX. 'V' for volt, 'A' for ampere, etc. SI standards recommends a space between the numbers and units just as you would for "5 apes" rather than "5apes". If you're using MathJAX as you have for your voltages then the norm is that variables are italicised and units are not. e.g. $ U = +6 \ \mathrm V $ gives$ U = +6 \ \mathrm V \\$. – Transistor Apr 10 at 16:05
• Why don't you use the block diagram of the closed loop configuration?Draw the block diagram and the equations between Vs,Vin and Vout and it will answer all your questions. – Miss Mulan Apr 10 at 16:07
• What is your math and EE background? You are asking about stability (oscillations). A proper answer requires the knowledge of several college-level courses. For a hobbyist, just trust that most opamps are stable under most conditions. The datasheet will usually warn you if there are stability risks and how to avoid them. – Mattman944 Apr 10 at 16:13
• It's connected from the output to the negative input so, by definition, it's negative feedback. Is it stable? Not necessarily, in fact some (very old) op amp required a resistor on the loop or were stable only with some minimum gain (typically 5). Look for "unity gain stable" in the datasheet. Specific requirements are usually noted out – Lorenzo Marcantonio Apr 14 at 6:55
• The 6V supply has nothing to do with gain. | Assume that the output moves to Vin + 0.01V say - what will happen? – Russell McMahon May 5 at 11:30

## 1 Answer

This is covered in most basic op-amp tutorials but here goes:

• Assume Vout is zero on power-up.
• If VIN+ goes to 0.01 V then the difference between VIN+ and VIN- will be 0.01 V. The output will start swing to 0.01 × A where A is the op-amp's open-loop gain - typically 100,000 to 1,000,000.
• As the output start to increase the difference between the two inputs decreases due to the negative feedback.
• When the output rises to 0.01 V the difference between the inputs is zero. The output should stabilise at this voltage.
• Would the difference between the two inputs not increase since the inverting input is at 001 x A now? – Filthyscrub Apr 10 at 16:27
• @Filthyscrub do you know how to draw the block diagram of this closed configuration? – Miss Mulan Apr 10 at 16:28
• @Miss, I'm not using block diagrams to explain it and it complicates the issue. Post your own answer if you like but please don't confuse this one. – Transistor Apr 10 at 16:29
• It doesn't complicate it at all , actually it makes it more easy! – Miss Mulan Apr 10 at 16:31
• @Filthyscrub, notice that I said, "the output will start to swing to 0.01 x A. For an A of 100,000 that means it would start heading towards 1000 V but as soon as it starts the difference is decreasing. If all is well (the system is stable) it won't overshoot past 0.01 V. – Transistor Apr 10 at 16:32