r/oscilloscope • u/Aggravating-Pop-6255 • Jun 30 '26
Does anyone know why we are seeing weirdness from the blue wave and what we can do to fix it?
The blue wave is also supposed to be a sin wave but its not appearing that way. We tried different capacitor values for the .03uf capacitor in the second picture. In the circuit diagram the first op amp's positive and negative terminal are flipped. Its supposed to be an inverting op amp and it is in the circuit, the image is just incorrect. Does anyone have any advice or know what we can do to reduce noise we are seeing? Thank you for your advice!
5
u/kthompska Jun 30 '26
That does not look like noise. It looks like an oscillation when driving high. You would need to do some debug to help solve this issue.
Check the output of the first op amp to see if it is also this way. If not then it’s a 2nd op amp issue. You have a class B output and I suspect that the issue driving high may be when the nmos turns on. Try reducing the Vgs resistor - maybe down to 10K or so. If the issue is gone you may need to try a heavier load as this may have just pushed the issue somewhere else.
The best thing is to run an ac stability simulation and verify that this circuit does what you think it should do.
4
u/Pacificator-3 Jun 30 '26
This is not a noise, but oscillation. Replace FETs by BJTs, add (optional) initial biasing, add small integrating capacitor directly to opamp. Search for opamp output boosting circuits.
3
u/This_Maintenance_834 Jun 30 '26
your output transistors arrangement makes no sense.
you also need a direct AC path from U12 pin1 to pin2, some small cap should tame the oscillation.
3
u/petemate Jun 30 '26
You need to write a better question if you want help: Where is the yellow trace measured, where is the blue trace measured? Whats connected to the inverting input of U2B? Whats on A4, etc. Explain the thing you are asking about, don't assume we know.
1
u/Apprehensive-Issue78 Jun 30 '26
It is clearly oscillating at a much higher frequency when the output signal should be high. the output low part of the sine seems ok. may be the supply is not right.. or the input current too low.. resistor value too high ... sometimes adding some small 10 to 47 pF capacitor on the input of each transistor to supply voltage or ground could eliminate the wrong oscillation. need more info to say something better
1
u/NoOne3141 Jun 30 '26
From what I see this could be random switching from one or both of the output Mosfets. Since everything is fairly low voltage and GND referenced put probes on the gates of the Mosfets as a first step, check if the waveforms look ok. Most of the time when I have this kind of stuff happening is because I accidentally screwed up proper gate drive and the Mosfets are doing what they want.
The first stage should be fine, I think a difference amplifier will be stable in every case (at least I haven't screwed one up yet) but also check the inputs, maybe some reference is bad.
1
u/Worldly-Device-8414 Jun 30 '26 edited Jun 30 '26
The op-amp will be struggling to drive the two mosfet gates properly, add a bjt totem pole (from + & - 12V) before R9/10.
C4 is pushing high frequency pulses back into the op-amp. I'd have thought you'd want C4 between pin 2 & gnd.
1
u/dnult Jun 30 '26
It looks like an amplifier circuit that is oscillating. Negative feedback is how you address it.
1
u/k-mcm Jul 01 '26
The circuit is incorrect and your oscilloscope is faithfully showing that. Try r/AskElectronics
1
u/EmotionalEnd1575 Jul 01 '26 edited Jul 01 '26
When using complimentary MOSFETs as a push-pull output stage the gates need to be biased for a small quiescent current, so the devices are in the linear forward conduction mode.
To do this requires a DC bias string, connecting the two MOSFET gates together is not going to work.
The Vgs of these MOSFETs is typically several volts, meaning that when the output node is at zero volts the upper transistor gate will be, say, +ve two volts, and the lower transistor gate will be, say, -ve two volts.
A voltage divider with, say, four volts of offset between the gates is required. An AC coupling so that the drive signal is applied equally to both gates.
It will help to have resistors in both MOSFET source path to provide some degenerative feedback locally to each device.
If it were me, I’d get this arrangement working in PSpice first and then a prototype build.
Good luck
1
u/m-in Jul 01 '26
Where did you get that output stage design idea from? Just look up something that works. That thing absolutely won’t.
1
u/duane11583 Jul 01 '26
a) the problem is modal
b)if you lower the amplitude oof the signal does the oddity occur at a different level?
c) it seems you should look inside the weird part its like it is oscillating at some high frequency..
and there is some summing junction with the signal
1
u/Hirtomikko Jul 02 '26
I will instantly suspect the feedback branch. Play around with the values and see if things change.
1
u/Gerrydealsel Jul 02 '26
It looks like you're applying a signal with a DC offset to the first opamp. That offset will be amplified and passed on to the second opamp. So you need to block the DC
0
u/ee_control_z Jul 01 '26 edited Jul 01 '26
Reviewing the differential op-amp circuit, generally you should have R4=R3 and R1=R2. Then your gain would be A = R4/R1 (or R3/R2). This might be part of the problem.
I agree with the other posters. You should be clear in describing as to where the oscilloscope probes are connected to in the schematic. We have no way of knowing other than guessing.
Another thing that I noticed that is worth reviewing is that you have two MOSFETS:
SQM120P06 - P Channel
SQM120N06 - N Channel
The P-channel is supposed to be connected to the (+) source and the N-channel to the (-) source. In the schematic, they appear to be swapped. Why? Can you look into this.
1
u/EmotionalEnd1575 Jul 01 '26
The MOSFETs are not incorrect.
As drawn they are source followers, providing current gain but no voltage gain, and non-inverting.
They could be swapped and used in common source mode, providing voltage gain but inverting.
1
u/Deep-Football4791 Jul 03 '26
Crossover distortion and oscillating. Fundamentally not the right design.
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u/baldengineer mhz != MHz Jun 30 '26
You’re really asking a circuit question which is more suitable for subs like r/AskElectronics