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"Bipolar" Expo Converter question . . .
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ryktnk



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PostPosted: Sun Feb 09, 2014 3:45 am    Post subject: "Bipolar" Expo Converter question . . . Reply with quote  Mark this post and the followings unread

Hello

I am trying to figure out if it is possible to make an
expo converter that can work in positive [+] and negative [-] current source.

IE can swing both sides of GND.

Any tips gratefully appreciated.

Thanks
-ryk
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frijitz



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PostPosted: Sun Feb 09, 2014 10:33 am    Post subject: Re: "Bipolar" Expo Converter question . . . Reply with quote  Mark this post and the followings unread

ryktnk wrote:
I am trying to figure out if it is possible to make an
expo converter that can work in positive [+] and negative [-] current source.

Not directly, because the conversion is done by a transistor, and current can only flow one way through a transistor. But you can multiply the current by a bipolar voltage, or switch its direction to get a bipolar source, eg as is commonly done in through-zero FM VCOs.

Ian
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gdavis



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PostPosted: Sun Feb 09, 2014 11:57 am    Post subject: Reply with quote  Mark this post and the followings unread

Could you have two opposing converters, one to handle the positive range and the other for the negative? Kind of like Haible's mirror core VC-HADSR.
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ryktnk



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PostPosted: Sun Feb 09, 2014 2:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hello

Thanks very much for the quick response.

I wanted to avoid switching, but It makes sense
now about the current flow direction. ( still learning )

Will try and read up on voltage current
multiplying

Thanks
-ryk
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frijitz



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PostPosted: Sun Feb 09, 2014 6:39 pm    Post subject: Reply with quote  Mark this post and the followings unread

gdavis wrote:
Could you have two opposing converters, one to handle the positive range and the other for the negative? Kind of like Haible's mirror core VC-HADSR.

Sure, that's another approach. A similar one is the dual converter used by Henry Wamsley in his TZFM VCO.

Ian
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ryktnk



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PostPosted: Mon Feb 10, 2014 1:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

frijitz wrote:

Sure, that's another approach. A similar one is the dual converter used by Henry Wamsley in his TZFM VCO.
Ian


Thanks for the tip Ian.

I looked at his Quadratur oscilator on EDN, is that the one you are referring to ?

Quite interesting, but to my basic understanding it looks to be two separate
cores that are driven by the relevant current sources above or below GND.

Thanks again.
-ryk
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frijitz



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PostPosted: Mon Feb 10, 2014 8:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

ryktnk wrote:
I looked at his Quadratur oscilator on EDN, is that the one you are referring to

Yes. It has two separate converters, and one would neeed to add temperature compensation to each one for serious VCO work.

Ian
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ryktnk



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PostPosted: Wed Feb 12, 2014 12:50 pm    Post subject: Reply with quote  Mark this post and the followings unread

frijitz wrote:

Yes. It has two separate converters, and one would neeed to add temperature compensation to each one for serious VCO work.
Ian


Hello

Yes it looks like it would be hard to get both
sides of the converter to match perfectly, the same problem with
Jurgen's VC ADSR.

Any tips on multiplying the current with a bipolar voltage . . .
I thought about a current mirror, but it looks like it only sinks the current.
A bit out of my depth. Embarassed

-ryk
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frijitz



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PostPosted: Wed Feb 12, 2014 1:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

ryktnk wrote:
Any tips on multiplying the current with a bipolar voltage . . .

You can simply use an OTA.

Ian
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ryktnk



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PostPosted: Wed Feb 12, 2014 3:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

frijitz wrote:

You can simply use an OTA.
Ian


Ah ha !
Duh, wish I was that clever.

So the bipolar voltage is fed to V-IN, the current to I-ABC and the
bipolar current from the output.

Thanks very much.
-ryk
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frijitz



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PostPosted: Wed Feb 12, 2014 6:44 pm    Post subject: Reply with quote  Mark this post and the followings unread

ryktnk wrote:
So the bipolar voltage is fed to V-IN, the current to I-ABC and the bipolar current from the output.

Right. OTA current switches have been used in Tri-core VCOs for many years. I use an OTA multiplier for the current reversal my linear thru-zero FM VCO.
http://bridechamber.com/F_Teezer.html

Ian
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ryktnk



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PostPosted: Fri Feb 14, 2014 3:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

frijitz wrote:
ryktnk wrote:
So the bipolar voltage is fed to V-IN, the current to I-ABC and the bipolar current from the output.

Right. OTA current switches have been used in Tri-core VCOs for many years. I use an OTA multiplier for the current reversal my linear thru-zero FM VCO.
http://bridechamber.com/F_Teezer.html

Ian



That was my goal to make a smoother running
TZO . . .
I hope I am not reinventing the wheel, or reinventing the Teezer !

Thanks again
-ryk
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