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Why is this LM158 oscillating?

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.

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

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

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

Reply to
bitrex

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.

Reply to
whit3rd

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

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

Spell checkers :-( ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

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
Reply to
Tim Wescott

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...

Reply to
Peter

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