[sdiy] expo converter /vco questions

[Kyle Stephens]

Dan,

You can get CA3140s at Futurelec:

http://www.futurlec.com/Linear/CA3140E.shtml

If you're doing (presumably) an ASM style VCO, this is *the* part for the job. I forget the explanation in detail, but that the chip has an input impedance in teraohms (10^12, 12 zeros!) plays a role.

I wish I could use NPN pairs more myself as I have a bunch of LM394s, which are also available on the cheap from Futurelec, but for reasons already explained better than I could do, they're not swapable with PNP pairs.

Lots of Ray Wilson designs use NPNs for expo converters though, so have a look there.

Examples of using SMT tempcos can be seen here:

http://modularsynthesis.com/magicsmoke/mankato/mse.htm

And here too:

http://hem.bredband.net/bersyn/VCO/vco_quad.htm


_Kyle

--- On Tue, 4/21/09, Jerry Gray-Eskue <jerryge at cableone.net> wrote:

> From: Jerry Gray-Eskue <jerryge at cableone.net>
> Subject: RE: [sdiy] expo converter /vco questions
> To: synth-diy at dropmix.xs4all.nl
> Date: Tuesday, April 21, 2009, 8:38 AM
> 
> 
> -----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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