[sdiy] Filters

[harrybissell]

I wondered about this... but in reality its very simple.

Voltage drop across a diode changes very little with current... being
nominally
about 0.7V....

Vary the current through the diode by using a constant current source, or
sink...
or for a crude approximation... by varying the voltage through a series
resistor.

Measure the current flow through the diode... and calculate the resistance
using
ohms law.

Of course, the voltage across the diode does change a little, so you need
to account for that
as well...

Now I don't KNOW how the diode acts as a resistor... but Ohms law assures
us that it
does....

H^) harry

David Reichert wrote:

> Now that this has been mentioned, are there any good sources explaining
> how to implement diodes as transconductance devices?
>
> -----Original Message-----
> From: owner-synth-diy at dropmix.xs4all.nl
> [mailto:owner-synth-diy at dropmix.xs4all.nl] On Behalf Of Grant Richter
> Sent: January 2, 2004 10:12 PM
> To: Linium; jbv
> Cc: synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] Filters
>
> on 1/2/04 5:58 PM, Linium at intent at netpratique.fr wrote:
>
> > On Vendredi 2 Janvier 2004 14:50, jbv wrote:
> >>>> Is there a (easy) way to modify this VC-BPF design :
> >>>> http://www.cgs.synth.net/modules/cgs35_syntha_vcf.html
> >>>> to add VC-Q to it ?
> >>>
> >>> Replace the Q pot with a LDR+LED (and 1k resistor parallel to the
> LDR) ?
> >>
> >> Mmh... I guess there my be a slighly more hi-tech, OTA-based
> >> solution...
>
> That is called a "Sallen-Key" filter. Buchla implemented it using a
> Vactrol
> for the transconductor. Neil Steiner used diodes as the transconductor.
> The
> Korg MS-20 implemented it using an OTA.