Stop Oscillations on Amplifier

[wuchy143]

Attached is my schematic. I have a basic amplifier which is exhibiting an oscillation at 55555Hz. My DC voltage range is correct for my desired amplifier but cannot stop the oscillation. It's not huge peak to peak. Any thoughts?

 

[crutschow]

Your simulation doesn't show any oscillation. When does it oscillate?

Post your .asc file.

 

[wuchy143]

It oscillates when used on my PCB which I just spun and populated. In simulation it does not oscillate.

I attached my .asc.

I was hoping I could stick a cap somewhere to stop the oscillations. What about putting a cap on the output? That does stop the oscillations but it doesn't seem like the best solution.

 

[alec_t]

It's not huge peak to peak. Any thoughts?

How huge? You can expect slight high frequency ripple on the output because the feedback does not operate instantaneously, due to stray circuit capacitance.

 

[wuchy143]

How huge? You can expect slight high frequency ripple on the output because the feedback does not operate instantaneously, due to stray circuit capacitance.

Worst case peak to peak is 2volts oscillation.

 

[ronv]

Your input impedance is very high, so even 10pf from the output to the + of the op amp will make it oscillate. If it is on a "strip" board that would be easy to have. I don't know what frequency response you need or if you need the high input impedance, but you could try a 100pf from the + of the op amp to ground just to check it out.

 

[crutschow]

The square-wave response and AC Bode plot of your amp show no significant signs of oscillations, overshoot, or peaking, so the problem may be in the layout.

Do you have a 0.1uF ceramic decoupling cap directly from the V+ pin to ground?

Are all output signal wires and connections well away from the input connections?

 

[WTP Pepper]

You have no decoupling capacitors on the supply. Any small ripple on the supply due to many reasons (RF pickup for one, flourescent light emmisions, TVs are bad as are LW radio transmissions ) are fed straight back into the input via R2. I tried to induce a few stray parasitics on the sim but I couldn't get it to oscillate either.

Just for interest, what did you do with the remaining 3 opamps in the LM324? Leaving their inputs unconnected is also asking for problems like you maybe seeing. If they can float about then you will be getting current gulps as they switch state due their high input impedance.

Simulators like LTSpice are brilliant for circuit simulators even down the the device functionality with the correct Spice models. However you get what you pay for. It isn't a full blown layout simulator. That's when you start paying £thousands.

Try to learn spider wiring of such circuits on copper clad board using short wires and avoid vero/breadboard as unless you know what you are doing. Long tracks and wires just act as aerials picking up all sorts as said above. They look a mess, but the technique is effective. I have done a complete IF strip using such techniques with about 70dB gain @ 21.4MHz using this technique without issues.

PM me if interested as I may have some pictures. If other have pics then please post. I am not sure of the correct terminology of this construction technique, it's just what we call it at work.

 

[wuchy143]

The square-wave response and AC Bode plot of your amp show no significant signs of oscillations, overshoot, or peaking, so the problem may be in the layout.

Do you have a 0.1uF ceramic decoupling cap directly from the V+ pin to ground?

Are all output signal wires and connections well away from the input connections?

I'm using a .1uF butted right against the power pin of LM324. I zoomed in on the voltage rail(at the chip) and it looks very clean. Nothing nasty like I"m seeing.

I'm looking at the layout now. Exactly what should I be checking?

 

[BobW]

It appears that your amplifier has a gain of less than one. 20k/75k=0.27
The LM324 is only compensated for unity gain, so it's very likely to oscillate. You will either have to increase the gain, or else add additional compensation.

 

[wuchy143]

Hi Bob W,

What would I need to do for additional compensation?

 

[wuchy143]

It appears that your amplifier has a gain of less than one. 20k/75k=0.27
The LM324 is only compensated for unity gain, so it's very likely to oscillate. You will either have to increase the gain, or else add additional compensation.

Does anyone know of a more suitable op-amp for what I'm doing? Perhaps the same pinout so I don't need to re-spin?

 

[MikeMl]

Your simulation is not actually using the LM324 model, instead you are using the built-in UniversalOpAmp2 model, which is a much faster opamp than a LM324. When I resimulated with the LM324, I get overshoot and ringing (marginal stability). If I add >30nF of capacitance across the 30Ω Load, then it oscillates just as you describe.

The problem is the additional phase shift in the feedback loop caused by the Darlington, amongst other things. The faster opamp has a better phase margin, so is less likely to take off...

 

[MikeMl]

It appears that your amplifier has a gain of less than one. 20k/75k=0.27

Look again, the gain is 1+20K/75K.

 

[wuchy143]

Your simulation is not actually using the LM324 model, instead you are using the built-in UniversalOpAmp2 model, which is a much faster opamp than a LM324. When I resimulated with the LM324, I get overshoot and ringing (marginal stability). If I add >30nF of capacitance across the 30Ω Load, then it oscillates just as you describe.

The problem is the additional phase shift in the feedback loop caused by the Darlington, amongst other things. The faster opamp has a better phase margin, so is less likely to take off...

I was simulating with the LM324, switched to see the difference and forgot to switch back before I posted. Sorry for the confusion.

Thanks its starting to become clearer. I took out the capacitor on my output(real circuit i made...not a simulation) and now believe it or not it is still oscillating but MUCH less. I had a .033uF in there which made it very bad. Do you think I could now take care of this much smaller oscillating with a 100pF in parallel with the feedback resistor in real life?

 

[wuchy143]

also could you attach your sim? I'd like to paw around. Thanks@!

 

[Winterstone]

It appears that your amplifier has a gain of less than one. 20k/75k=0.27
The LM324 is only compensated for unity gain, so it's very likely to oscillate. You will either have to increase the gain, or else add additional compensation.

Yes - and more than that: The Darlington stage in the feedback loop introduces additional phase shift into the loop. Thus, positive feedback for some frequencies is not a surprise.

 

[MikeMl]

I was simulating with the LM324, switched to see the difference and forgot to switch back before I posted. Sorry for the confusion.

Thanks its starting to become clearer. I took out the capacitor on my output(real circuit i made...not a simulation) and now believe it or not it is still oscillating but MUCH less. I had a .033uF in there which made it very bad. Do you think I could now take care of this much smaller oscillating with a 100pF in parallel with the feedback resistor in real life?

Here is how I would do it. Note that I am driving a more capacitive load. Using an NFet allows for a nice compensating network. Note that it oscillates with C1=1p. You can get everything from underdamped to overdamped by changing C1.

 

[crutschow]

It appears that your amplifier has a gain of less than one. 20k/75k=0.27
The LM324 is only compensated for unity gain, so it's very likely to oscillate. You will either have to increase the gain, or else add additional compensation.

It's the equivalent non-inverting gain that's used to determine stability and that can never be less than 1. So an op amp compensated for a gain of 1 will always be stable with any value of inverting gain, even less than one.

 

[crutschow]

Your simulation is not actually using the LM324 model, instead you are using the built-in UniversalOpAmp2 model, which is a much faster opamp than a LM324. When I resimulated with the LM324, I get overshoot and ringing (marginal stability). If I add >30nF of capacitance across the 30Ω Load, then it oscillates just as you describe.

The problem is the additional phase shift in the feedback loop caused by the Darlington, amongst other things. The faster opamp has a better phase margin, so is less likely to take off...

Why did you add >30nF of load capacitance. That's a lot of stray load capacitance.