What happens when shorting your opamp output? - Page 1
Forum section: electronics / beginners

author: Saimoun
author_rank: Frequent Contributor
timestamp: March 27, 2021, 04:56:13 pm
content: HiSo I have been wondering, if you have a biased opamp, doing say audio, and a big cap at the end. If the output gets shorted for one reason or another, what happens?This is the circuit:https://www.circuitlab.com/circuit/ck6t3bz8ju2t/circuit-test-load/(see also attached picture)It seems that then the only load left is the output resistor R34, and the voltage at the actual output is zero (makes sense - it is grounded).I have an opamp which tends to oscillate when adding capacitive loads, and I was worried since shorting the output in this case would add a 33uF capacitive load, is that correct or did I miss something?I have made some real circuit measurements and I get similar results than the attached simulated results, and I could not get the opamp to oscillate (strangely).Any thoughts?Thank youSimon

author: magic
author_rank: Super Contributor
timestamp: March 27, 2021, 05:42:36 pm
content: The important thing here is that both the output and the inverting input are connected before R34, so the resistor somewhat isolates the opamp from the capacitor. Such a series RC network always has less than 90° phase lag, as would be the case with a capacitor alone.It also helps that the capacitor is large enough that its reactance is almost zero at high frequencies. It's almost like a short, on microsecond timescale lots of current can pass through it with only a small change in voltage.Things tend to be worst with low ESR capacitors somewhere in the range 100pF~100nF.

author: Zero999
author_rank: Super Contributor
timestamp: March 27, 2021, 06:33:16 pm
content: It depends on what the op-amp is doing. It might oscillate, if there's no signal, or could current limit. In this case, when the capacitor is initially shorted to 0V, the output is at half the supply voltage, so it will most likely current limit, then it might oscillate.

author: Saimoun
author_rank: Frequent Contributor
timestamp: March 28, 2021, 08:41:52 am
content: Thank you both, I am not sure I fully understand your replies though.I get that R34 helps, and that the feedback loop is connected *before* R34 (i.e. R34 is not in the loop) as you said magic.But I would guess because the capacitor is big it makes the potential oscillation problem bigger? See the attached note from my opamp's datasheet.

author: Jan Audio
author_rank: Frequent Contributor
timestamp: March 28, 2021, 01:12:36 pm
content: Nothing happens, you just have made a integrator ( low pass filter )Is the capacitor on your power line also short circuit ?

author: magic
author_rank: Super Contributor
timestamp: March 28, 2021, 02:08:36 pm
content: This is your problem in one picture. Green is the output, blue is the feedback.90° delay and some attenuation going from the output to feedback.90° delay plus inversion and some gain going from the inverting input to the output.Result: effective 360° phase delay and unity gain around the loop, hence instability.If you increase capacitance, attenuation increases and oscillation moves to a lower frequency where the opamp has more gain. At some point it runs out of gain and can't sustain oscillation.And if you wonder why I took feedaback from the right side of R1, that's because LTspice ideal opamp has zero output impedance and 90° phase so it drives any load without oscillation. Real amplifiers fall somewhat short of this ideal and that's why they struggle with reactive loads.Isolation resistor improves the situation by reducing phase lag seen by the inverting input.

author: Saimoun
author_rank: Frequent Contributor
timestamp: April 08, 2021, 11:28:31 am
content: Thank you all for your replies, and apologies for my lack of answer.The subject got a bit too technical for me I thinkI'll see when I get the actual boards and test from there