How Elektor stabilized the 3046 (was: uA726 replacement)

[Edu Silva]

Hi, Anders and list!

The following text was translated from an old issue of Elektor
(brazilian edition). I put the schem at my website, link at the end of
the page. It describes the log converter from a sweep generator, using a
heated 3046. (Please, forgive my english...)

"The transistor T5 act as a temperature sensor (-2mV/*C). The difference
beetween VBE (0,6V) and the voltage drop on R20 is amplified by A8, and
excite the current sources T6 and T7. Every time VBE > V20, a high
current will flow by this two transistors, heating the IC. When the
temperature reaches the value set by P9, the current trough T6 and T7
goes down.

This way, a stable condition is achieved, in which the dissipation over
T6 and T7 keeps the temperature of the IC under very narrow limits. As
soon as the IC reaches its operacional temperature the LED D2 ('log
sweep not avaliable') turns off: voltage at the input of A8 will be
beetween -5 and 0V."

Note that T5 (temp sense) is transistor (12,13,14), and pin 13 is
connected to the -5V point. It must be done this way.

Yes, it's a log converter, but can easily be adapted to work as an expo
conv. The schematic can be found at this page:

http://audiolist.cjb.net/download/

Hope this helps.

Edu Silva
ES2 Audio

http://audiolist.cjb.net