Rbe test circuit?

[Rene Schmitz]

At 09:11 16.07.99 +0200, Martin Czech wrote:
>:::Or measuring Vbe with variing collector current?
>:::Any better ideas?
>
>since the rbe effect is mostly in higher Vbe, and you go for expo
>applications... yes , I guess this would be an idea. I would also measure
>Ib, and I would try to have Ie constant, like in the moog circuit.
>
>Of course, one should check this with an almost ideal tranny,
>any MAT02 left ? I have two which I have never used...
>
>m.c.
>

After thinking about it, I come to the conclusion that the collector must
be open,
since the Early effect would change the Vbe too.
I think perhaps the way would be to draw the Vbe/Ib courve on log paper.
(Did I say paper?!, I mean gnuplot or something!) For an ideal device I'd
expect a straight line (just Ebers Moll). In real devices for higher base
currents there should be a deviation from the straight line. But this would
be difficult to measure, since I think the deviation perhaps is already
swamped in the error bar. At some hundred uA of base current the input
resistance is maybe still in the range of some hundred Ohm.
Btw, Would it be perhaps possible to get the value for R(be) from the
h-parameters?
Like extrapolating h(ie)vs Ic for Ic -> inf. If Ic (thus Ib) is very large,
and therefore the B-E diode's dV/dI very small, the remaining value would
be the static Rbe (???).

Bye
 René




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