I have a simple noninverting amp around a LM158. I am using a 158, not a 358, for the extended temp range. The feedback resistors are 2k2 and 2k0 (the 2k0 goes to GND). So a voltage gain of just under +2. Supplies are +/-15V, well decoupled with 10uF ceramics. I am finding that if I feed in a 100Hz sinewave of about 1V pk, the output shows a superimposed ~1MHz sinewave during the bottom 30% of the bottom half-cycle. I can kill this oscillation by sticking say a 1uF ceramic from output to GND... Looking at the schematic e.g. here one sees a straight push-pull output stage, with a 50uA sink (a current mirror presumably) to the -ve rail, to allow the output to go all the way down. But I am not going anywhere near the -ve rail. I am going to -2V only. Now look at page 18 of that PDF. It shows 6.2k resistors from the o/p to the -ve rail. Why? And sure enough, putting a 10k from the o/p to the -ve rail removes the oscillation. I am going to put in a 4k7 to be sure... This op-amp is supposed to be unity gain compensated... The data sheet for the STM version (which I am using) is different but it shows the same 6.2k resistors. I'd appreciate any input on this...

Outside the FB loop and after the pulldown resistor. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net

Why is this LM158 oscillating?

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Peter 12 years ago

Tim Wescott wrote

OK - thank you both.

It turns out that the load I am driving is 300k and a 1k series resistor is going to introduce a negligible error in this case.

I've been doing both analog and switching power supplies since the

1970s but have never got my head around poles and zeroes :)

I can certainly see the principle behind taking the DC feedback from after the resistor - that's obvious. But the feedback cap is more subtle...

Re the other question - a LM358 was widely used for basic audio circuits years ago. It's not really suitable for anything decent, of course. My present application is a very simple DC one, but I am testing with an AC waveform because it's an easy way to check the voltage range.

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Jim Thompson 12 years ago

Which makes me puzzle over the stability of OTA's... their open-loop output impedance is very high. ...Jim Thompson

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bitrex 12 years ago

The LM13700 will oscillate like mad if it's used with voltage feedback and no external compensation capacitor.

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whit3rd 12 years ago

You never use them without a load resistor (they're always tested with one). And, you don't always care about feedback stability, because they don't always get used with feedback. They perform operational tasks, without being 'operational amplifiers' in the traditional sense of high-input-impedance, low-output-impedance with high voltage gain.

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Jim Thompson 12 years ago

OK. Figures. My thought processes are aimed at accurately modeling that behavior.

I have a whole lot of behavioral schemes rattling around in my head at the same time... analog AND digital... release soon after the move is com plate. ...Jim Thompson

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Jim Thompson 12 years ago

Spell checkers :-( ...Jim Thompson

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Tim Wescott 12 years ago

Is the six feet (or whatever) of 50-ohm coax going to be in the product, or is that just how you're measuring things?

If it's just a development artifact, get a decent scope probe!

Tim Wescott Control system and signal processing consulting www.wescottdesign.com

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Peter 12 years ago

Tim Wescott wrote

In the actual installation it will be driving about 2m of a shielded cable, Raychem type 55, impedance unknown to any degree, plus some input capacitance of an indicating instrument, so I think that coax is a reasonable thing to do.

Funnily enough had I used a 10x scope probe the problem would have never surfaced...

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Why is this LM158 oscillating?

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