Oscillation

[udip]

Hi,


Can someone tell me how should I know if an opamp is
oscillating? what should I see on scope?
(I'm talking about high frequency oscillation, not audio range)
Should I check with high frequency input signal?


Thanx,

Udi.

[Mr Evil]

Just look for a high frequency sinewave on the output. It could be at many MHz and quite a low level, so set the scope to a small timebase.

If it's only slightly unstable then it might need an input signal to set it oscillating. A square wave is good for that, since it contains a broad range of frequencies. Also look for ringing on the square wave at the output: A lot of ringing is a sign of instability.

[udip]

Thanks.

[udip]

Hi,


Does it mean that I need a high bandwith oscilloscope in order
to know if my preamp/amp is doing ok?? what should be
the bandwith of scope - 1,10,100 mhz??? cause its far
beyond audio range, how much does it really matters if
my amp is oscillating at 1 mhz, etc?? how does it affect
the sound?


Thanks,

Udi.

[Mr Evil]

A 1MHz scope is not fast enough. 10MHz should catch most problems. 100MHz is more than fast enough for just about everything.

As for the problems caused by oscillation: Increased heat, reduced headroom, impossible to set bias, RFI (the oscillating equipment will both radiate interference and conduct it back out through the mains). It may be immediately audible as a high pitched squeal, or if not it may be indirectly audible as fuzzy treble, crackling etc.

[JohnW]

Faster OPAMP’s like the AD797 like to oscillate around 500MHz - this oscillation can be less then a 1mV, making it very hard to see on a normal scope.

Elevated / unstable noise floor in an audio band FFT normally a sure sign things are unstable higher up in the spectrum...

I use a RF spectrum Analyzer to check Audio OPAMP’s are not singing there little hearts out up in the MHz region.

You would be surprised how many audio products I've come across that have HF oscillations - value gear is even worst - people seem to have forgotten the art of "Grid Stopper" resistors - small, say 100R resistors fitted in series as near as possible to the Grids.

I would say 40% of value gear I've measured is unstable in RF band. Values, like MOSFET’s are HF devices - poor PCB layout, combined with high drive impedances almost guarantee oscillation issues.

If you’re using the new fast breed of OPAMP’s – be weary of oscillation issues – hunting down a HP3585 40MHz Spectrum analyzers a good start…

Best of luck,

John