[sdiy] expo converter /vco questions

[Oren Leavitt]

Howdy Dan,

Replies in line:

Dan Snazelle wrote:
> 
> I have a few (well quite a few) basic questions about Expo Converters and VCO's
> 
> 
> 1. when i look at a vco and it has a PNP pair, can I replace that with an NPN pair?
>
No - not unless you modify the rest of expo converter circuitry to drive 
  an NPN pair, and use a VCO or filter core that use an opposite current 
flow (source/sink).
Basically flip the polarity of everything else around the transistor 
pair - lots of work...way easier just to use the PNP pair <grin>

> 2. Is there any advantage (other than cost) to using one or the other (npn or pnp pair?) it seems NPN pairs are cheaper and more common.
> 
I agree that NPNs are more abundant and convenient. The inexpensive 
CA3083 NPN arrays for sale from some surplus sources can perform 
surprisingly well as expo converters when compared to a premium device 
such as the MAT02 - and you have a few extra "free" transistors to use 
wherever too.
Ian Fritz has mentioned these in the past.

> 3. has anyone had good luck with the SSM2164? and once again, is it to replace an NPN or PNP pair?
> at 6 bucks it seems like a good soloution (As you could get an expo AND a vca out of one chip right?)

I have not used this device - no advise to offer from my end here.

> 4. Do you HAVE to use an lm311? could you use say an lm2903 or 339 or even a standard opamp?
> 
Comparator ICs are not quite as "plug n play" as are op amps (in most 
cases). The 311 (and the dual 319) have faster response times than the 
339, 393, etc. This is crucial in sawtooth core oscillators that need to 
completely reset the integrator with half a microsecond.

> 5. in electronotes they had mentioned that using a discrete Schmitt trigger worked better than a 3080 or a 555...is there a modern best Schmitt trigger for vco use?
> 
The two-transistor Schmitt trigger was desirable because of its speed 
back in the day before fast IC comparators were available. The MiniMoog 
and Moog 921 oscillators are good examples.

> 6. has anyone had much luck with those surface mount tempcos i have heard about?
>
I have not tried them myself. Others have used them with good results.

> 7. what is a good replacement for the 3140 if i dont have one? electronotes said a 351 but that was a long time ago.
> 
The design requirement here is very high input impedance and high slew 
rate. The CA3140 did this very well back in its time. Still a very good 
choice if you have a supply of them.
The are excellent modern op amps available that will meet or beat the 
CA3140.

> 
> also...can anyone recommend a good NPN pair Triangle VCO?

No immediate recommendations myself. The Buchla oscillator core might be 
a good one to play with. It has an expo converter and a current mirror, 
so you have both NPN and PNP pairs.

> 
> thanks so much
> 
Happy soldering!
- Oren
> 
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