VCO help needed.

[kjarger at jkjelec.com]

Callum wrote: 

> My other query is with the LM3046.  Apart from the limits imposed by the
> required connections, does it matter which transistors in the package are
> used.  I have left out the one with its emitter connected to the substrate,
> as some directions I found indicated the substrate should be connected to
> the lowest voltage in the external circuit.  When I connected the substrate
> to ground there seemed to be no difference in the result.


The transistors on the 3046 are all laid out on a common substrate, 
and the manufacturers recommend that this substrate pin be tied to the most 
negative point in the circuit.

They say that the transistors will not be isolated properly from the 
substrate otherwise.  I guess this would cause signal leakage between 
transistors, high current draw and generally unpredictable behavior 
because the PN junctions inherent in the substrate/transistor
boundary would sometimes go forward biased.
I would highly recommend following their advice.  It may be fine for now 
left floating, but you never know...

Also, I guess it's ok to leave the unused transistor connections floating, but 
I would tie the bases and emitters at least, probably to ground, 
except the substrate transistor's base, which I'd tie to V-.

Call me obsessive if you'd like.

Regards,

Kyle Jarger
JKJ Electronics
Visit our MIDI/CV website http://www.jkjelec.com