Strange Oscillation 3

[tandy2]

hello,

im using an opa551 op amp as a voltage follower to a tle2024 amp. Its non-inverting, and has a negative feedback, with a 1k resistor on the feedback and input. Ive put a scope on the output and im getting almost 1v of oscillation (at about 1 meg) without any voltage going into the circuit, and i cant think whats causing it. It has 2nf capacitance and 250 ohms on the load, but it can supply 200mA of current so thats not a problem, and I have the same amplifier in another part of the circuit with 220nf capacitance on the load, so that shouldnt be a problem either.
Anyone have any idea whats causing this and how I could stop it?

Thanks

 

[VEBill]

Curious - What frequency is the oscillation?

Check the data sheet and application notes to see if that OpAmp requires a compensation capacitor.

 

[tandy2]

oscillation is at 1Meg.

The data sheet does show a circuit for driving high capacitive loads, but like I said, it copes fine with 220nf on the load, but not with 2.2nf.

 

[VEBill]

Sorry I missed the "1 meg" the first time.

Assuming that you're not dealing with HF signals, then put a suitable capacitor from output to inverting input. Choose one that will provide a good amount of negative feedback at 1MHz while not significantly affecting your desired signal.

Still need to confirm why it is oscillating in the first place.

Lead lengths and dressing okay?

 

[OperaHouse]

I've seen that happening in circuits by where the ground is connected. Ground was shaped in a U that made a perfect tuned circuit. Also, just length of coax on the input can make it happen. I started working for a company and on this test amplifier they hung a cap on the output. I asked what that was for and they said it would sing without it. I put a 100 ohm resistor in series with the fet input. That was enough to dampen it, never a problem after that. Just step back and look at the layout a little differently and the solution will come to you.

 

[tandy2]

Thanks for the help.
The tracks arent to long, about 1 mm nominally, and dont cause problems elsewhere so i dont think these are the problem.
I can see what your both saying, but im a bit reluctant to add in any new components as this will mean a change to the layout, meaning lots of paper work. I was hoping to just be able to alter a few resistors or caps, but I guess I'll have to make greater mods to the circuit.
Ive spoken to some colegues who came up with similar solutions to these as well.

ah well, better get typing my change requests

thanks again

 

[VEBill]

You might be able to get some expert advice from the OEM application engineers. By definition, they should know (or be able to determine) exactly what is causing the oscillation. They would likely need to see the schematic and PCB layout for that part of the circuit.

Another tip - toss a frequency counter on the oscillating output and pay attention to the exact frequency while poking around in the circuit (with your finger assuming it is low voltage). The idea is to make sure that the frequency of the 'oscillation' isn't exactly the same as the AM radio station's transmitter just down the street (in other words, EMI), and to make sure that you can shift the frequency a bit by touching the components that you believe are involved.

 

[maged541946]

Hi, sometimes an easy way to see what is going on is to connect the scope to the supply rails, here we have 2 op amps, if the supply has osscilations then most likely the problem is undesired coupling that need de-copling capacitors an CHIP's power pin, with say 0.1u ceramic disc or any surface mount good quality caps. If the oscilation is only at that output, the feedback is within that chip, the de coupling cap may help, also the series resistor Mentioned above by OperaHouse (2 resistors , one Between each chip pin and the rest of the circuit, also sometimes changeing the vale of resistors (keeping the same ratios for gain or else) will change the impeadences that input and output see and the effect of stages on each other.

 

[MaddMan]

Singing Amplifier:

I started working for a company and on this test amplifier they hung a cap on the output. I asked what that was for and they said it would sing without it.

I have a stupid question... does the "singing" frequency have anything to do with the bandwidth of the amplifier? Or is this caused by something else?

 

[itsmoked]

I've been flumoxed by commercial radio transmissions many times with OPAMP circuits.

 

[walker1]

http://yoink.ece.olin.edu/datasheets/OPA551.pdf

page 12 shows a circuit dealing with capacitive loads.

The document also states, that even though the chip is unity gain stable, it is optimized for gain=5. The above circuit 'fakes' a high frequency gain around 8, which I have seen making other op-amps work at unity gain, even if their specifications required, say, gain>5. (LF357 for instance)

The shown system will, of course, increase the output noise some, as the HF gain is higher.

 

[elfgriper]

tandy2,
Consider reducing both the feedback and input resistors. Just short them out and see if this eliminates the problem,
-elf

 

[jsolar]

You do not have enough phase margin with your non-inverting unity gain heavy load. You should keep the load > 300 Ohms, including your feedback load (which looks like a capacitor feedback to the inv terminal). If you can spare adding a 50 Ohm resistor in series with the output keeping the feedback at the output pin, you might solve your problem. If not, it is time to open up you favorite feedback compensation textbook and add a capacitor and a resistor. It will not take much to fix the problem. By the way, the information here is really contained in several of your earlier replies.
Best Regards,
John Solar