[volt-nuts] 7081 AC buffer *again*

[David C. Partridge]

Ed, thank you, that pencil trick is one I don't know.  One of the problems with being "self taught".

Mind you, if it is FB round the op-amp, then anywhere in the FB loop might well show as a culprit :(

Regards,
David Partridge
-----Original Message-----
From: volt-nuts-bounces at febo.com [mailto:volt-nuts-bounces at febo.com] On Behalf Of Ed Breya
Sent: 30 January 2013 04:57
To: volt-nuts at febo.com
Subject: Re: [volt-nuts] 7081 AC buffer *again*

I agree with JL - it's either picking up an available frequency, or making its own. Oscillation can happen with transistors, especially any common-base amplifiers or emitter followers that don't have enough degeneration - but that is usually in the VHF range. For MHz-type oscillations, look for opamp loops that went unstable for some reason - bad grounds or bypassing can do it.

An old trick for VHF is to poke around with the tip of a wood-sheathed lead pencil - the partially-conductive graphite core and the lossy wooden capacitance to an even lossier hand would sometimes damp oscillations and point directly to the problem, so to speak. For lower frequency you need brute force - try a small screwdriver, a meter probe lead, or tweezers held in the hand, and just poke around on various nodes, without shorting anything out. If that's not enough, a small RC series damper - say 1000 pF and a few hundred ohms - with a clip lead to ground should show some results. All you're looking for is some effect - it should either reduce or aggravate the problem when you are around the problem circuit. You will naturally be injecting all kinds of line frequency and RF interference into any high-Z circuits, so ignore that and just look for the effects at the target frequency.

Ed
_______________________________________________
volt-nuts mailing list -- volt-nuts at febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.