matched transistors

[Ian Fritz]

Hi Renee --

> (Besides I still believe that you don't need a tightly matched pair in
> expos. The fact that you can build precise and stable VCOs with cheap
> parts, suggests that its a myth.)

I've seen this too. The main reason for using a matched pair is to cancel
the T-dependence of the reverse saturation current (I_s prefactor in the
expo equation). Good matching essentially guarantees that the prefactors are
the same and therefore cancel at all temperatures. If the matching isn't
perfect then the prefactors could be unequal, but their T-dependences could
still cancel, for example if the prefactors are related by a multiplicative
factor caused by a slight difference in area of the two devices.

> However there are good reasons towards LM394, MAT0x etc. when you are
using
> these in the audio path. And in expos it eliminates the need for adjusting
> Rbe compensation, which can save time when you do "mass production".

I agree. Still, isn't a high frequency tracking trimmer usually needed
anyway?

  Ian