Re: beta at low currents

John Larkin wrote:

On Tue, 21 Apr 2009 18:03:31 -0400, Spehro Pefhany
<speffSNIP@xxxxxxxxxxxxxxxxxxxxxxx> wrote:


On Tue, 21 Apr 2009 14:30:27 -0700, the renowned John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:


Anybody got wisdom on using transistors at very low (like, picoamp)
collector currents? What happens to beta?

I'm thinking of a common-base current source in this specific case. I
need a controlled current source from low nA to low pA, low noise but
not especially accurate... it would be inside a bigger loop. Driving a
transistor common-base would work nicely in my situation.

I did some googling, but all the good stuff seems to be in non-public
journals.

John

IME, which only extends down to nA, you get lower beta. The obsolete
2N929 was recommended as having decent beta at 50nA Ic.

Fundamentals of Solid-State Electronics By Chih-Tang Sah indicates
that you might expect a beta of around 1 at 10pA and worse at lower
currents, but maybe modern transistors are not quite the same.


Since base current has shot noise, a beta of 1 is very bad news.

I guess I'll have to try something else. But maybe I'll test a few
modern high-beta transistors just to see.



Maybe a job for an optoisolator?


Hmmm... some low-capacitance PIN photodiodes might be interesting.
Again, I'd have to see how bad the shot noise would be.

John



Try selected part types made by Zetex...ZTX1051A, ZTX689A, ZXTN25015DFHTA, ZTX457, FCX458TA, ZTX696A.
.

Relevant Pages

  • beta at low currents
    ... Anybody got wisdom on using transistors at very low ... What happens to beta? ... I'm thinking of a common-base current source in this specific case. ...
    (sci.electronics.design)
  • Re: beta at low currents
    ... I'm thinking of a common-base current source in this specific case. ... transistor common-base would work nicely in my situation. ... 2N929 was recommended as having decent beta at 50nA Ic. ...
    (sci.electronics.design)
  • Re: beta at low currents
    ... I'm thinking of a common-base current source in this specific case. ... IME, which only extends down to nA, you get lower beta. ... but maybe modern transistors are not quite the same. ...
    (sci.electronics.design)
  • Re: beta at low currents
    ... I'm thinking of a common-base current source in this specific case. ... IME, which only extends down to nA, you get lower beta. ...
    (sci.electronics.design)
  • Re: beta at low currents
    ... John Larkin wrote: ... Beta goes kaputt unless you have devices made at a lab, small, very low defect rate and very low parasitic current. ... You'll need a pretty big sense resistor, like 10**10 ohms, because you need to drop at least 50 mV across it to get the shot noise to dominate the Johnson noise. ...
    (sci.electronics.design)
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