[sdiy] Measuring Rbe

Ian Fritz ijfritz at comcast.net
Wed Jun 15 15:28:59 CEST 2011

I usually just draw a straight line through the linear part of the curve 
and figure out its slope.  You don't want to include the curved data at low 


At 03:46 AM 6/15/2011, sdiy wrote:
>Thank you Ian,
>for calculation: I calculate the delta of Vce and Ib from point to point
>(measured in 1k steps). Dividing the deltas give me the Rbe. Then i sum the
>Rbe up and divide through the numbers of measuring points to become the
>average. Right?
>Am Mittwoch, 8. Juni 2011, 19:19:41 schrieben Sie:
> > At 10:14 AM 6/8/2011, sdiy wrote:
> > >I followed the description above. Emitter on ground. Basis at +10V with
> > >varying the R in one k steps from 1k to 30k. Collector open. Then
> > >calculating deltaVce / deltaIb to become Rbe
> > >Measuring an CA3046 at 26degCelsius I calculated Rbe at about 7Ohm. That
> > >is near what Rene measured.
> > >I will give the other figures i did when the method is confirmed.
> > >
> > >But i am not sure if i understand it right. I am wondering why i can
> > >measure Vce when the collctor is open? Can anyone shet some light on
> > >this?
> >
> > The procedure is correct, I believe.  Both Rene and I did a number of
> > devices this way a "few" years ago.
> >
> > To measure the extra series resistance Rbe, you want to measure Vce vs Ie
> > and calculate the slope at high current, where the slope is dominated by
> > Rbe.  The measurement needs to be done at zero collector current so that
> > any spurious resistance Rcb does not contribute. Ie is the same as the base
> > current Ib, because the collector is open. So the slope of Vce vs Ib gives
> > you the desired result.  Seems a bit odd, but I think the way to look at it
> > is simply that the collector floats to whatever voltage is necessary to
> > satisfy the transistor characteristics at zero collector current.
> >
> >    Ian
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