AW: Substrate temperature stabilized tra
The Old Crow
oldcrow at Access.Mountain.Net
Thu Jun 8 00:50:00 CEST 1995
On Wed, 7 Jun 1995, Haible_Juergen#Tel2743 wrote:
> US$20,- is a bit steep for just the heart of an expo converter!
> If it were a complete temperature stabilized pair for that price, ok.!
> But where is the advantage over a simple and cheap 3086 ?
> If we are able to build a good thermostat with a MAT-04, why not
> the same with a 3086 ?
> Just wondering ...
> (I believe that the transistors are "better" than in a 3086, but I
> thought 3086 transistors were good enough, when the temperature
> effects would be cancelled.)
According to National Semiconductor specs and appnotes, the LM3046
(LM3086), when used in a feedback temperature stabilized circuit, has an
exponential conformity of 0.25% from 20Hz to 15KHz, which is just fine as
far as I'm concerned. The circuit of AN-299 shows an emitter bulk
resistance compensation network that feeds approx. 8.23mV into the
(normally grounded) +input of the integrator. From this, I estimated the
bulk resistance to be about 5.6 ohms, based on the maximum current that
will be seen by the exp. converter junction at the top octave [10v max.
control voltage is divided down by a 10.5k/250ohm to 0.23v, which is the
transistor's specified saturating Vbe], which is around 1.46 mA. (8.23mV /
1.46mA = 5.6 ohms approx, the 1.46mA is from 6.9v / 4.7k ). From this I
gather that the frequency tends to go flat in the higher octaves.
Of course, I could be wrong, but this is what I gathered in a glance at
the various data sheets. For what its worth, I think I'll go ahead and
build a sawtooth VCO using an LM3046 just to run some tests. I'll post a
jpeg of the circuit on my homepage sometime in the next few days for
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