From: John Woodgate
Subject: Re: Help with audio o/p transformer feedback to inv OP Amp
Date: Mon, 13 Jan 2003 09:01:33 +0000
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Mon, 13 Jan 2003 13:09:09 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that Jeff wrote
(in ) about 'Help with audio o/p
transformer feedback to inv OP Amp', on Mon, 13 Jan 2003:
>Im trying to add an output transformer to a conventional inverting Op Amp
>stage - but add the transformer pri and sec into the feedback path, so gain
>Have normal feedback resistor, plus another from secondary o/p to inverting
>Transformer windings are phased to not invert.
>Depending on ratio of these two resistors, the cct will oscillate. Cold end
>of primary and secondary is grounded, as is + input of Op Amp. Using TLO71
>with bipolar +/-8V supply. Gain is only 2. Typical speech band tones used.
>Can get it to work, but transformer feedback resistor has to be much greater
>than the Op Amp feedback resistor. I dont see why this has to be so.
>Is there some special way of coupling back the transformer so there can be
>greater percentage of transformer feedback, as opposed to device feedback?
Apart from the oscillation, what is the turns ratio of your transformer?
If you have a gain of only 2 from input to primary, due to the feedback
around the op-amp itself, and the transformer has a turns ratio of less
than 1:2, you have insufficient open-loop gain for the overall feedback
to do anything.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
PLEASE do NOT copy news posts to me by E-MAIL!