[sdiy] Measuring Rbe

[Ian Fritz]

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