Temperature control for transistors/crystals

[Rick Jansen]

This month's Elektuur (Dutch Elektor) magazine has a temperature
stabilization circuit for crystals, which we could use very well 
to keep dual transistors in VCO's temperature stable.

It uses a Darlington transistor as both the heater and the sensor.
80% of the time the darlington acts as a heater, 20% of the time
as the temperature sensor. It is switched between these functions
with 4 switches in a 4066 ic. A 555 timer, an opamp, two 8V
regulators and a short-circuit-proof 2x9V 3.3VA transformer are 
main the other components.

The crystal is kept at a temperature of 40-50 degrees (adjustable).
It's locked away together with the darlington in a styrofoam box.
The transistor and crystal are simply joined, no extra heat"sink"
to act as a flywheel are used.

You could use this circuit to temperature-stabilize dual transistors
in exp-generators. By far the best way to do this is substrate
heating, i.e. heat the silicon chip the transistors are on directly.
You need at least two extra transistors on the wafer to do this, 
one transistor as the heater, the other as the sensor. The problem
here is that transistors that really have good log characteristics
are usually duals (MAT-02, MAT-03) and single-sourced components.
The only multi-sourced transistor array I know is the 3046/3086,
which has 5 transistors, but the transistors in there are not the 
best for exp purposes. There is a (single-sourced!) MAT-04, which has 
4 transistors, and is very promising as a substrate temperature
stabilized part. But, it is a single-sourced part. If you base a
design on it you may find yourself in the "uA726 situation". The
uA726 was a dual transistor made by Fairchild, with on-chip 
temperature control. The uA726 was used in various Moog, Roland
and Kobol VCO's and also in the Elektor Formant VCO's. Fairchild 
does not exist any more, uA726 was single-sourced and thus it is 
highly unavailable today.

With the above circuit you could use the best and the best available 
dual transistor of the day in an exp converter.

Source: Elektuur (Dutch Elektor) - oktober 1996

Rick Jansen
__
rick at sara.nl   http://www.sara.nl/Rick.Jansen
_____________________________________________
S&H's a module and s&h's looking good