[sdiy] Expo converters ( uh-oh, I started reading EN... )

[Ian Fritz]

Hi Tom --

At 01:56 AM 11/28/05, Tom Arnold wrote:

>First question pertains to the VCO described in ENS76 option 1A, also EN37
>page 4 Fig F.  For those without EN handy this is your run of the mill two
>NPN's from a CA3046 expo with a third transistor from the 3046 acting as
>active compensation for bulk resistence.  Concerning that 3rd transistor,
>does it have to be matched to the pair in the expo itself?  What I have in
>mind for playing with is to use an Option 1 VCO which is based around the 
>LM394
>and add the compensation circuit.  I figured I'd at least thermally couple
>the compensation transistor, but matching wasnt planned.

The bulk-emitter-resistance correction is usually quite small, and with the 
LM394 it will be very small.  So it is not critical that the third 
transistor be matched or thermally coupled.  In fact, you can use a simpler 
circuit just using a signal diode or a diode-connected transistor:
http://home.earthlink.net/~ijfritz/sy_cir2.htm


>My second question is about another Expo is in the same EN.  This ( EN37 
>page 3
>Fig E ) is the good'ol Arp pnp/npn pair but with a pnp/npn pair
>compensation circuit.   I havent seen this used much besides, well, Arp and
>never with the compensation.  I was thinking of trying a THAT340 since it
>seems to fit the bill and is pretty darn affordable.  Besides the fun of
>playing with it I'm worried that it just doesnt work well enough to persue,
>mostly based on not seeing anyone else using this particular circuit.  Is
>that fear proven, or is this just not a circuit people have played with
>much?

It's hard to match pnp/npn pairs -- that's the reason this configuration is 
not so popular any more.


>Can you tell my fixation is on VCOs right now?

Ah, excellent.  Now you truly have the bug.


   Ian Fritr