[sdiy] ASM1 - MAT & caps

[Ian Fritz]

Tom et al. --

I would just point out that the textbook Is equation is for an idealized 
(and simplified) model.  If this equation is assumed to hold, then an 
imbalance in Is will not induce temperature dependent effects (temperature 
terms have already been cancelled out of the equation).

However, the equation assumes there are no defects in the material. For 
example, if there were a dislocation cluster in one of the two devices, 
then the temperature-dependent carrier density would differ on the two 
sides. In this case, the NA ratio in the textbook formula would have to be 
replaced by by a complicated expression giving the carrier-density ratio in 
the two transistors. This would give a temperature-dependent Is ratio and 
therefore a drift in the converter pair.

A better method of matching is probably possible, but it would have to 
involve measurements as a function of temperature. For example: bias the 
transistors with large resistors to the power suppply and identical base 
voltages and look for changes in the differential collector voltage vs 
temperature.

   Ian



At 09:34 PM 2/6/2002, Tom May wrote:
>Let's see how this shakes out.  Gray and Meyer say:
>
>       q A Dn ni^2
>Is =  -----------
>          Wb NA
>
>where
>
>    q is q (constant)
>    A is a contant area
>    Dn is related to electron diffusion
>    ni is the temperature-dependent intrinsic carrier concentration
>    Wb is the width of the base (which varies with the terminal voltages,
>       at least)
>    NA is a constant related to doping
>
>What we're interested in is the temperature dependence of the ratio
>Is1/Is2 of the two transistors.
>
>Is1   q A1 Dn ni^2     Wb2 NA2       Wb2 NA2 A1
>--- = ------------ x ------------  = ----------
>Is2      Wb1 NA1     q A2 Dn ni^2    Wb1 NA1 A2
>
>A2, A2, NA1, and NA2 won't change so in theory we get:
>
>Is1                  Wb2
>---  proportional to ---
>Is2                  Wb1
>
>So we're down to the temperature dependence of Wb.  At this point it's
>not clear whether this is "good enough", or whether matching Is1 = Is2
>is better.  We're getting into second (or third) order effects here
>with the base width modulation and temperature variation of Wb, which
>I don't recall having been mentioned before, so I would guess Vbe
>offset doesn't really matter.  I would obviously defer to empirical
>evidence to the contrary, however.
>
>Tom.