Strange Oscillation
Strange Oscillation
(OP)
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
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





RE: Strange Oscillation
Check the data sheet and application notes to see if that OpAmp requires a compensation capacitor.
RE: Strange Oscillation
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.
RE: Strange Oscillation
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?
RE: Strange Oscillation
RE: Strange Oscillation
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
RE: Strange Oscillation
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.
RE: Strange Oscillation
RE: Strange Oscillation
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?
RE: Strange Oscillation
RE: Strange Oscillation
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.
RE: Strange Oscillation
Consider reducing both the feedback and input resistors. Just short them out and see if this eliminates the problem,
-elf
RE: Strange Oscillation
Best Regards,
John Solar