[sdiy] expo converter /vco questions

[Neil Johnson]

Hi,

(some answers inline)

Jerry Gray-Eskue wrote: 
> 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.

The SSM2164 is a current product, available from Digikey amongst other places, although you'll have better luck with the lead-free parts, e.g. SSM2164PZ.

> 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.

Actually, its easier to do it with surface mount tempcos as you can glue then directly to the component you're trying to track.  And being small they respond quicker to thermal changes due to having smaller thermal mass.  I did this with a surface mount tempco from Farnell epoxied to the side of a 2SC1583.

> 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.

CA3140 - BiMOS single op-amp
LF351  - FET input op-amp

Cheers,
Neil
--
http://www.njohnson.co.uk