Re: Vbe stuff



Winfield Hill wrote:

Eeyore wrote...
Jonathan Kirwan wrote:
Jim Thompson wrote:
Jonathan Kirwan wrote:

All the same, it makes some sense to me. Shockley's equation develops
from an assumption that the forward current is entirely due to
minority carrier diffusion in neutral regions, I think. I need to
think more about it, but on first cut it sounds like the right
direction.

Large ratios of doping level are precluded by (primarily) breakdown
voltage and forward resistance considerations.

But isn't forward resistance a valid part of this measurement being
discussed?

I'm confident of it. I've never seen rbb specified for devices like
the MPSAs but I have a vague recollection of seeing a figure of ~
1k somewhere once for small signal parts. If so, the 100uA would
account for 100mV in the resisitve part of Vbe.

Whoa, 100mV is a serious error!

Look at the numbers. The OP did mention 100uA btw.


Let's talk 10mV; that'd be 10uA
of base current, which would correspond to about 2mA of collector
current. Since the measured characteristics don't deviate from
the ideal by that much by 2mA, I'd imagine rbb' must be under 1k.
But we're probably in the right ballpark. A 1k rbb' corresponds
to 4nV of lowest en noise at high collector currents. I don't
know the lowest en value for mppa parts, but I imagine they're
in the 2 to 5nV region; certainly not below 2nV (250 ohms).

One of the few parts I've seen En for was a Rohm part ( 2SB737) which was ~
600pV. That had an rbb' of ~ 10 ohms IIRC.

Graham

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