[sdiy] expo converter /vco questions

[Dan Snazelle]

thanks for the answers..very helpful

i think i worded the part about replacing the PNP pair incorrectly. what i was wondering was if its possible to reconfigure the entire circuit to work with the opposite polarity. and from what you said it sounds like it is but it also sounds like a lot of work

the ssm2164 is a vca chip that roman sowa has used to create a vco.

thanks!



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----------------------------------------
> From: jerryge at cableone.net
> To: synth-diy at dropmix.xs4all.nl
> Subject: RE: [sdiy] expo converter /vco questions
> Date: Tue, 21 Apr 2009 10:38:00 -0500
>
>
>
> -----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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