About NATIONAL APP NOTE #299 VCO

Mikko H. MHELIN at tne01.tele.nokia.fi
Wed Apr 24 12:24:27 CEST 1996


This is a kind of old thing, but as I was browsing a data sheets for
CA3046 I found a note "The collector of each transistor of the CA3045 and 
CA3046 is isolated from the subsctrate by an integral diode. The substrate
(terminal 13) must be connected to the most negative point in the external
circuit to maintain isolation between transistors and to provide for
normal transistor action." Maybe this explains why AN-299 VCO didn't
work as it should have, and why all 3046 based VCO won't oscillate 
at low frequencies (as Gene pointed out). EN #73 VCO option 1b in EN
Preferred Circuits Collections also mentions that sustrate pin should
be connected to -15V thru 10k resistor. With AN-299 VCO there's a problem
as an absolute max. rating for collector-substrate voltage is 20 V, and
the collector of Q6 is connected to 15 V, so maybe 20k/10k divider
should be used instead. I haven't build that VCO yet as I'd like to
know if this feedthrough through substrate was the reason for original
problems with that VCO design. 

-Mikko



More information about the Synth-diy mailing list