From: Winfield Hill
Subject: Re: Help with audio o/p transformer feedback to inv OP Amp
Date: 12 Jan 2003 19:27:28 -0800
Organization: Rowland Institute
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> I'm 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 better linearity. Have normal feedback resistor, plus
> another from secondary o/p to inverting input. 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 don't 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?
Yes, Jeff. Try a capacitor in parallel with the opamp's feedback
resistor. The transformer has excess phase shift at high frequencies,
and select this to bypass that pathway at high frequencies. You'll
also need the opamp path for dc, otherwise the opamp output will have
dc offset and drive dc current through the transformer. Bottom line,
dc and hf feedback must be from the opamp's output, mid ranges from
the xfmr. We show how one way to do this in our book, AoE page 249.