Author Topic: OTA variable resistor  (Read 788 times)

Kipper4

OTA variable resistor
« on: December 28, 2017, 05:09:21 AM »
Hey guys

Has anyone made good use of an ota variable resistor to make a phaser,wah,vibe.

The circuit in question is Fig 13 here.

https://www.nutsvolts.com/uploads/magazine_downloads/11/May%202003%20Ray%20Marston%20-%20Understanding%20And%20Using%20OTA%20OP-Amps.pdf


Maybe an envelope detector to control the Iabc
maybe an lfo to make a trem.

All contributions welcome.

What would you use it for?

Rich
Jellybean dodging since 2012.
Smoke me a Kipper. I'll be back for breakfast.

EBK

Re: OTA variable resistor
« Reply #1 on: December 28, 2017, 07:10:38 AM »
At first glance, it appears to be a very expensive vactrol replacement, but I'm a throw-more-parts-at-it-for-academic-purposes type, so I like the idea a lot.  :icon_wink:

(It's potentially quicker than a vactrol though, so there are some possibilities....)
« Last Edit: December 28, 2017, 07:12:19 AM by EBK »
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Fender3D

Re: OTA variable resistor
« Reply #2 on: December 28, 2017, 07:24:54 AM »
Has anyone made good use of an ota variable resistor to make a phaser,wah,vibe.

I'd think it's easier to get an all-pass-filter out of an OTA than trying to match those Rx pads with any op-amp (or transistor) external circuit.
An offset control, on the OTA, is mandatory and it might mess up the Rx formula...
"NOT FLAMMABLE" is not a challenge

EBK

Re: OTA variable resistor
« Reply #3 on: December 28, 2017, 07:30:40 AM »
it might mess up the Rx formula...
Might it mess it up in a useful way? 
No affiliations. If I glowingly mention specific merchants or products, it is because I like them without having to be paid to like them.

Fender3D

Re: OTA variable resistor
« Reply #4 on: December 28, 2017, 07:37:54 AM »
Might it mess it up in a useful way? 

Dunno... but since you need to change R1 and RA values to achieve 0V or whatever offset voltage you'd need, then Rx formulas change...
"NOT FLAMMABLE" is not a challenge

Kipper4

Re: OTA variable resistor
« Reply #5 on: December 28, 2017, 08:01:50 AM »
I'll breadboard it with a pot for R1 Ra (assuming this is whats meant by the offset)
Running on a single +9 supply with V/2 biasing.

I'll inject some volts to Iabc and see what Rx I get.

Depending on the results will largely determine what It might be useful for with the single supply.

with nsl32 vactrols around the 2.5 GB pounds it seems like the chip might be as economical as a vactrol.



Jellybean dodging since 2012.
Smoke me a Kipper. I'll be back for breakfast.

samhay

Re: OTA variable resistor
« Reply #6 on: December 28, 2017, 10:55:47 AM »
I think quite a few of us have got (temporarily) excited about this application at some point or another.
However, with these circuits you still can't escape the issues you typically have with OTAs in other configurations - they aren't terribly quiet and they get gross when you hit them with too much signal.

I'm a refugee of the great dropbox purge of '17.
Project details (schematics, layouts, etc) are slowly being added here: http://samdump.wordpress.com

PRR

Re: OTA variable resistor
« Reply #7 on: December 28, 2017, 01:04:56 PM »
In the single-OTA gyrator, the emulated impedance has one side grounded. Our standard phasers use a floating variable resistor.

Other configurations are possible. Indeed we can rig the gyrator to make a fake "inductor" and vary it. Against a resistor, that would give the right result. But a brain-pain to work-out the values for an L-R phaser from our known-good C-R phasers.

The OTA can directly be rigged as an inverting low-pass of variable cutoff, which mixed with direct would do the right thing. Again some brain-pain needed.

The very typical scheme in the Marston paper multiplies the OTA input hiss by 200. This could be changed, but that also changes the overload point. "200" works because the input overload is ~~20mV and 200X allows 4V signal at the output, a workable level. But the balance is tight.

ElectricDruid

Re: OTA variable resistor
« Reply #8 on: December 28, 2017, 03:16:40 PM »
Has anyone made good use of an ota variable resistor to make a phaser,wah,vibe.

Short answer, no.

Slightly longer answer, like PRR said, there's usually another better way to use a OTA for each of the applications you mention (VC all pass stage, VC bandpass filter or resonant lowpass filter). Taking something that wasn't designed to be variable and making it variable by hacking in a circuit which is sort-of pretending to be a resistor is the long way around and doesn't help. So in all the years I've been aware of it, I've never found a use for it. Other people may have more imagination, of course...

Tom

Kipper4

Re: OTA variable resistor
« Reply #9 on: December 29, 2017, 05:21:33 AM »
Thanks guys for the input.
I fudged around with it on the breadboard and came to the conclusion to file it in the been there tried that.
I did manage to get resistances from 30k~40M+ (O/L)

Marginally better when the a 12k>+9v diode biasing. Lowered the resistance a lot.

As most of you have pointed out if there was any mileage in it you guys would have been all over it already.
Sometimes you just got to try stuff.
lessons learnt.

Jellybean dodging since 2012.
Smoke me a Kipper. I'll be back for breakfast.

garcho

Re: OTA variable resistor
« Reply #10 on: December 31, 2017, 04:31:33 PM »
^ good to wire up the data sheet apps on popular ICs anyway. Everyone has already tried everything; basically that's true. BUT occasionally someone has a flash of inspiration and figures out how to use one of those old tools for some new task. No old tools, no flash of inspiration.
"...and weird on top!"

PRR

Re: OTA variable resistor
« Reply #11 on: December 31, 2017, 09:31:06 PM »
Federico S. "Fender3D"

Your avatar www. wizardinside. it/foto/schemi/ avatar.jpg is causing a login pop-up when this page loads. (Probably every page you post on.) (Also pages on FSB.com.)

It would be good to fix the permissions on the server or change the image to a non-private URL.

Kipper4

Re: OTA variable resistor
« Reply #12 on: December 31, 2017, 09:34:09 PM »
Thanks for pointing that out PPR. I was beginning to wonder what that was about..
Jellybean dodging since 2012.
Smoke me a Kipper. I'll be back for breakfast.