[sdiy] Feedback in filters [was: Re: From a commercialstandpoint...]

Simon Brouwer simon.o at brousant.nl
Thu May 5 10:50:33 CEST 2016


In a transistor ladder filter the variation in emitter-bias voltage is small,
and a couple of 100 millivolts of emitter-collector voltage would be sufficient
to keep the transistors out of the saturation area. So biasing the transistor
pairs, say, 0.5V apart should do the trick already.

Best regards

> Op 4 mei 2016 om 13:58 schreef Gordonjcp <gordonjcp at gjcp.net>:
> On Wed, May 04, 2016 at 10:37:06AM +0100, Richie Burnett wrote:
> > Yes, you can make some filters self-oscillate by feeding one of
> > their outputs back to the input with phase-inversion if necessary.
> > But it is not a sure fire method that works with all filters.  It
> > depends on the maths of the filter.  It works for a moog 4 pole
> > cascade because there is 180 degrees of phase shift at the cutoff
> > frequency, and that's how the resonance is implemented inside the
> > filter normally anyway.  Conversely if you feed back the BP output
> So, if I made an eight-pole filter would I need to feed the resonance back in
> phase with the input then?
> Let's leave aside how high the positive rail would need to be, for all those
> diodes :-D
> -- 
> Gordonjcp MM0YEQ
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