Circuit Board Update 4/23

Joachim Verghese jocke at netcontrol.fi
Wed Apr 24 12:13:55 CEST 1996


On Tue, 23 Apr 1996 gstopp at fibermux.com wrote:

>      The exponential 
>      converter uses a pair of NPN's, so that means that a couple of 
>      hand-matched 2N3904's should work fine. That's what I'm using right 
>      now, and they're not even matched.

What does Electronotes say about this matter? According to my experience,
the two transistors of an exponential voltage-to-current converter need
not be matched in the normal sense, i.e. for equal Vbe and/or Beta.

The first transistor is used to provide an offset voltage for the
exponentiating (second) transistor. This offset voltage contains
the temperature dependent factor of the transistor's reverse collector
cut-off current. If this factor is equal for the two transistors,
and they're always at equal temperature, the effect will be completely
cancelled. (Note that this does not eliminate the temperature dependence
of the exponential scale factor (0.33%/K) which is why you still need
a tempco resistor or similar).

In other words, the two transistors should ideally be matched for similar
temperature charateristics, and have good thermal coupling.
This is perhaps easiest to achieve by using integrated transistor pairs
or arrays. If you use discrete devices, I'd imagine you'd get good results
as long as you provide adequate thermal coupling. (ARP used to put brass
clips around their TO-92 pairs). It might also help if the two transistors
are off the same manufacturing lot.

Any differences in Vbe or Beta will be corrected for by the frequency
offset trimmer of the VCO.

-joachim



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