[sdiy] Negative content on half wave rectifier

[Harry Bissell]

LOL now how did I guess you did that ?

been there.....

H^) harry


----- Original Message -----
From: Justin Owen <juzowen at gmail.com>
To: Harry Bissell <harrybissell at wowway.com>
Cc: SDIY List <synth-diy at dropmix.xs4all.nl>
Sent: Fri, 22 Jul 2011 11:25:47 -0400 (EDT)
Subject: Re: [sdiy] Negative content on half wave rectifier

From: Harry Bissell [harrybissell at wowway.com]
Subject: Re: [sdiy] Negative content on half wave rectifier

>Justin, you're not looking at the opamp output are you ? That's not the output
>point for the circuit. (but it might explain the readings you are getting...)

Bah! Yes, that's exactly what I'm doing. <slaps forehead>

I also put the scope on 1V/Div rather than 5V/Div and I'm getting readings that make more sense now - down to -0.08V with those settings and er... taking the output from the right place.

In my defense ;) one good thing I did discover and then found in that ESP app note was using the resistor to ground from the (op amp) output to hard clip the +V portion of the wave - although I'll have to go back and check that now I've been corrected on where the output really is.

Thanks much,

Justin






----- Original Message -----
From: Justin Owen <juzowen at gmail.com>
To: SDIY List <synth-diy at dropmix.xs4all.nl>
Sent: Fri, 22 Jul 2011 09:43:26 -0400 (EDT)
Subject: [sdiy] Negative content on half wave rectifier

Hello,

I was recently warned off using a single 1N4001 as a half wave rectifier so I started playing with op-amp based versions. I settled on the 'Improved Precision Half Wave Rectifier' in Fig. 3 here: http://sound.westhost.com/appnotes/an001.htm

This also shows up on P191 of Jung's op-amp cookbook.

Jung quotes that, on an AC signal, you'll see a negative swing on the output of approx. -0.6V (which I guess is an inverted diode forward voltage drop?) - in reality I'm seeing between 0.8V and 1V depending on the diodes I use (1N914s were best).

Is there a way to completely get rid of the -V content or at least improve these figures - so you're swinging between 0 and V?

I have tried biasing the +v terminal but, for input summing of AC and DC signals it's not a solution. To compensate for the +1V bias I made the 0-5V CV swing between -1 and 4V but obviously that -1V portion of the CV is now getting rectified and lower voltages don't smooth up and down like they do without any bias (but with the -V content) - they 'bump' on and off, I guess as the diodes turn on/off - so it's causing more problems than it's solving.

For the record, I can live with the negative content - but it is bugging me.

Thanks,

Justin



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--
Harry Bissell & Nora Abdullah 4eva



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Harry Bissell & Nora Abdullah 4eva