PAiA VCO

[René Schmitz]

Hi Ian, Jim and List.

I have tried to get behind the secret of this circuit. As a first 
step I redrew it, and at some point the circuit started to look 
familiar to me. Now I see much clearer. Infact it looks like a version of
the exponential current source in the National Semi. datasheets for the
LM13600. Which even features an analysis of this circuit. According to
their maths the expo scale factor is independant of T. (So the gm is really
the tempco, not the diode.) At least theoretically this will cancel,
although in practice thermal coupling considerations apply. (Which was the
reason why I used four trannies on one die when I did a solid state
compensation.)

Bye,
 René



At 08:16 08.08.00 -0600, Ian Fritz wrote:
>Hi Jim --
>
>Yes, this is all very curious. I hope to find time to wire this thing up and
>measure its temperature dependence. I notice there is no drift spec listed.
>
>  Ian
>
>----- Original Message -----
>From: "patchell" <patchell at silcom.com>
>To: <synth-diy at node12b53.a2000.nl>
>Sent: Monday, August 07, 2000 8:55 PM
>Subject: Re: PAiA VCO
>
>
>>     I just took a good look at the circuit.  I also redrew it , which I
>find
>> helps a bit.  But after I looked at it I saw something else that I find
>> curious.  In the FAQ it said it was using the diode in the linearizer for
>a
>> temp sensor.  I don't think so.  In order to use a diode as a temp sensor,
>you
>> need to maintain a constant current through it.  The way the circuit is
>set
>> up, very little if any current will be flowing through that diode.  In my
>> opinion, the tempco would be generated by the temperature dependence of Gm
>of
>> the transconductance amplifier.  Even if the diode were forward biased,
>it's
>> tempco would be canceled out the tempco of the Gm.  I depend on this for
>my
>> own electronically compensated exponential converter.
>>
>>     Time to break out the superstrip, I guess.  I am going to get to the
>> bottom of this puzzel.
>>
>
>
>
-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159