[sdiy] Matching transistors in "diode ladder" filters

[ASSI]

On Sonntag 20 April 2008, Aaron Lanterman wrote:
> I told my student to use CA3046s to get some cheap pairs (since I
> have a bunch), but if he could use garden variety diodes for some
> that would be convenient.

If you make the diodes out of monolithic transistor arrays anyway, I 
would short collector and base and not leave the collector open as the 
S100 schematics show.  This should give a much better log conformance 
over more decades of current and the requisite hfe matching should be 
guaranteed.  The dynamic resistance is different and will probably 
require to change the capacitors a bit.  Using garden variety diodes 
should work in principle, but the linearity would often be quite a bit 
worse than a BE diode from a transistor, signal diodes with not too 
high breakdown voltage should be best.  There are some dual diode 
arrays in series connection (meant for input protection) that should 
help to cut down on the number of packages (I think there is a TI quad 
array with eight seperate leads, but with Schottky diodes IIRC).


Achim.
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