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

[Grant Richter]

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

Also note: In the mixed THAT array, the NPNs are matched, the PNPs  
are matched, but look at the offset spec between the two types.
PNPs and NPNs arrive at a different Vbe even with identical  
monolithic doping.
The only way to get 1 mv. Vbe match from PNP to NPN is hand matching.  
Very tedious and finicky.

The three published types of high frequency compensation are (and  
correct me if I misspell the names).

Franco type: A resistor in series with timing cap. Actually  
compensates for comparator reset time, but can "swamp" Rbe error  
somewhat.

Hemsath type: That's the one with the transistor driving the joined  
emitters.

Rossum type: This is the simpler one with a diode or diode connected  
transistor. Not mentioned in regular Electronotes, but a brief mention
in one of the mid-month newsletters (how did Bernie do it?). Easy for  
NPN pairs, more difficult for PNP pairs.