newbie asks: current/true voltage contolled vco?

[Harry Bissell]

OK I'll bite (as will others i'm sure..)

The 3046 has a unique "feature". Since all of the transistors are
fabricated on the same
slab 'o silicon... they are not truly isolated from each other. The
substrate (slab) is connected to each transistor through what seems like
a reverse biased diode junction... that is if the substrate is the most
NEGATIVE point in the circuit... Pin 13 is also the "substrate"
connection and if it is left floating it can queer the whole chip. It
should be tied to the most negative point in the circuit. (without too
much study) you probably fixed this problem with what you bridged....

Most VCO's are at thier hearts CCO's.  The voltage is changed to a
current, usually by the expo converter (which MUST function in current,
as it is the current relationship in the transistor which has the
exponential characteristic).  This usually charges or discharges a
capacitor.

The Book is correct in that the OTA is controlled by a current, which
the expo converter supplies. The resistor in series with the bias input
is not required in normal operation... but it will limit the possible
current during startup (when the supplies might do ANYTHING) or
malfunction... The current just goes happily in one end and out the
other.

Resistors convert current flow to voltages... but in this case we aren't
looking at voltage.
The drop across R2 will be a voltage proportional to current... BUT WE
DON'T CARE
(as long as it isn't so big we run out of power supply voltage...)

If you look at the voltage at LM13600 pin 1, you will see that it
doesn't change very much with frequency... its a forward biased diode
junction (inside)...  its the current that
is making the difference...

BTW if you have a VERY sensitive current meter (can read down to low
microamps...)
you can put it in series and watch the current flow into pin 1...

H^)



Michael Schmid wrote:

> I'm building the modular of penfold's book
>
> http://aupe.phys.andrews.edu/diy_archive/s
> hematics/synths/penfold/penvco.html
>
> "The VCO", he writes, "remains a current rather than a true voltage
> controlled type." Sorry for my newbieness, but what's the difference.
>
> Btw: it didn't work the first few times, but as I touched the pcb with
> my fingers by accident, it functioned. I bridged some solder jonts of
> IC2 (CA3046) ...
>
> ... and it worked. Guess which: Pin 3 (emitter of Q1 and Q2, which are
> not used anyway) is bridged with 12 (base of Q5) and 13 (emitter of
> Q5).
>
> Maybe someone can explain this.
>
>
> Thanks a lot,
>
> Michael
> (Could you please stay in an electronicnewbiespeech?)
>
>
>
> --
> michael schmid //  linz // austria