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 Forum index » DIY Hardware and Software » Thomas Henry designs
Exponential Current Sources
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fonik



Joined: Jun 07, 2006
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PostPosted: Tue Dec 22, 2009 5:21 am    Post subject: Exponential Current Sources
Subject description: Thomas' VCO designs
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we often see an NPN and a PNP as part of an exponential VCO in thomas' projects. only occasional there is a configuration using i.e. LM394 (matched NPN pair). why?
i don't know that much about maths so i wonder if i could easily interchange them?

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fluxmonkey



Joined: Jun 24, 2005
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PostPosted: Tue Dec 22, 2009 8:06 am    Post subject: Re: Exponential Current Sources
Subject description: Thomas' VCO designs
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fonik wrote:
we often see an NPN and a PNP as part of an exponential VCO in thomas' projects. only occasional there is a configuration using i.e. LM394 (matched NPN pair). why?
i don't know that much about maths so i wonder if i could easily interchange them?


along the same lines, is there a single-packagecomplementary NPN-PNP transistor pair suitable for these designs? i've always heard it was difficult to hand-select pairs w/ matching characteristics.

b

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Sound



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PostPosted: Sun Jan 03, 2010 11:47 pm    Post subject: Re: Exponential Current Sources
Subject description: Thomas' VCO designs
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fonik wrote:
we often see an NPN and a PNP as part of an exponential VCO in thomas' projects. only occasional there is a configuration using i.e. LM394 (matched NPN pair). why?
i don't know that much about maths so i wonder if i could easily interchange them?


In "The VCO Chip Cook" and in "Making Music With The 3080 OTA" the NPN/PNP configuration and the PNP pair configuration are used in same VCOs so are interchangeable.

In these books the PNP pair configuration is always specified as a precision exponential current source, better than the PNP/NPN configuration.

About interchange a NPN pair configuration by a PNP/NPN configuration, I have not seen it nor tried it, but I guess maybe its possible placing the PNP/NPN mirrored at the NPN/PNP configuration, I mean placing the emitter where is the collector, feed with +V where it is fed with -V and vice versa. I would say adjusting the resistor output will be enough.


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Sound



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PostPosted: Sun Jan 03, 2010 11:55 pm    Post subject: Re: Exponential Current Sources
Subject description: Thomas' VCO designs
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bbob wrote:

along the same lines, is there a single-packagecomplementary NPN-PNP transistor pair suitable for these designs? i've always heard it was difficult to hand-select pairs w/ matching characteristics.

b

There is a THAT chip with two PNP and two NPN, but as the PNP pair matched (or NPN) configuration is specified as better i only would use it.

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rumpofsteelskin



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PostPosted: Mon Jan 04, 2010 3:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

i assume you are talking about the one pnp / one npn exp converter and the slightly more complicated one with two pnps and an op-amp??

yes they are interchangable. Hand matching transistors can be a pain, but is not "hard" per se. I have used both circuits in different vcos

Using a monolithic matched pair of transistors minimises differences in the betas of the two and gives you less frequency drift when they change temperature. Normally if two seperate transistors are used they are epoxied together to avoid temperature differences causing differences in beta. The circuit with the two pnp transistors and op-amp will give you the least temperature drift, the circuit with one npn and one pnp will give you more.

the one pnp and one npn circuit is often used in places where you need an logarithmic response but its not particularly critical i.e. volume, whereas the slightly more complex circuit with an op-amp and two pnps is generally used where the accuracy is more important - pitch

f you have not already read it this explains everything in more detail -
http://electronotes.netfirms.com/s019.pdf
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