[sdiy] expo converter /vco questions

[Jerry Gray-Eskue]

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Dan Snazelle
Sent: Tuesday, April 21, 2009 7:33 AM
To: synth-diy at dropmix.xs4all.nl
Subject: [sdiy] expo converter /vco questions




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?

All the Current flows in a PNP and NPN transistor are Opposite, that is to
say the  do the same thing but one is the Reverse of the other. Anything you
can do with a PNP you can do with an NPN -BUT- all the rest of the circuit
has to be built for the opposite polarity. The short answer is you cannot
use one for the other.

You can use two discrete transistors for a pair, typically you use matched
transistors and glue them together ( so they are running at about the same
temperature ) , common setup use 2n3904 for NPN pairs and 2n3906 for PNP
pairs.

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.

Using a circuit that has a compatible current flow to the other equipment
(modules) is simpler to design.

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 believe this is an OBSOLETE part, the data sheet refers to it as a Low
Cost Quad Voltage Controlled Amplifier. This is not a transistor pair, it is
4 complex devices on the same chip.


4. Do you HAVE to use an lm311? could you use say an lm2903 or 339 or even a
standard opamp?

The LM311 is a comparator with some special pins, If the special pins are
not in use OR not doing anything special - any number of devices with
compatible supply voltages can be used. The same is true of most opamp
devices.

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?

There are hundreds of ways to setup a Schmitt trigger, what is "best"
depends on what you want, voltages, output, cost, speed, crispness etc.
Without a schematic it is very difficult to make any recommendations.

6. has anyone had much luck with those surface mount tempcos i have heard
about?

Tempcos are used to try to correct Temperature Drift, they work best when
Attached to the device they are tiring to correct so they are at
approximately the same temperature as the device. This is hard to do with
surface mount. Tempcos also need to match the device they are correcting,
Over correction is just as bad or worse.

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.

I do not recognize that part number and cannot turn up a data sheet on the
Web for this one, If it is a transistor pair see the above comments.

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

thanks so much


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