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WTD: Small signal NPN, HFE >= 200 at Ic < 2uA...

Discussion in 'Electronic Design' started by Greg Neff, Apr 3, 2007.

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  1. Greg Neff

    Greg Neff Guest

    ....surface mount, Vceo >= 30V. Low frequency.

    That Ic is not a typo. I have to operate with an Ic in the range of
    0.01 to 2 microamps. Not my idea, but I have been tasked to determine
    the feasibility. Considering NXP PMBT6429 and Fairchild KST5089, but
    my Ic is still way off the left side of the DC current gain curves.
    Any recommendations?


    ================================

    Greg Neff
    VP Engineering
    *Microsym* Computers Inc.
     
  2. John  Larkin

    John Larkin Guest

    BCX70K? Or a fet!

    John
     
  3. John  Larkin

    John Larkin Guest

    The BCX70 is very well behaved. High beta, not too fast (tends to not
    oscillate on its own), beta banded, low leakage, cheap.

    John
     
  4. Joerg

    Joerg Guest

    Hey, thanks. This very minute I was looking for a lower noise transistor
    to drive a laser, has to be really low in noise below a few hundred
    Hertz. The old 3904 doesn't really cut it but it looks like the BCX70
    does. And it comes in SOT-23. Nice.
     
  5. ON BC847C/848C?
     
  6. Guest

    BF862 is used in car radios. The IDSS is tighter than most JFETS. I
    bought 50 on ebay from this vendor:
    <http://cgi.ebay.com/BF862-SMT-J-FET-Junction-
    Transistor-25mA-20V-50_W0QQitemZ170096508822QQihZ007QQcategoryZ4666QQssPageNameZWDVWQQrdZ1QQcmdZViewItem>
     
  7. Joerg

    Joerg Guest

    I just wish there was more data on those kinds of gems. In my case I
    need the low 1/f at around 10mA, they only give us data for 0.2mA.

    Still mulling the BF862, a really nice JFET. en is only given for 100kHz
    but sure looks nice, 0.8nv/rtHz (typ). Also, Digikey has about 40,000 of
    these in stock and there has got to be a reason. Unfortunately not
    pin-compatible with BJTs. Why did they...? Arrgh.
     
  8. Guest

    Not to worry, Greg. Many, if not most, small-signal transistors
    will work very will in the 10 to 2000nA region. Their beta will
    be somewhat lower, and their f_T will be substantially lower at
    a given current (by sqrt(1/Ic), but most of the other parameters
    will work fine. For some BJTs, this holds down to the pA region.
    Vbe drops by 58mV/decade at room temp, but you already knew that.
     
  9. Eeyore

    Eeyore Guest

    You still use the 3904 ?

    Aren't more modern parts like the BC549/559 somewhat better (notably higher beta
    IIRC) yet the same price ?

    SMT versions available as BC849/859.

    Graham
     
  10. Eeyore

    Eeyore Guest

    BC849C ?
    http://www.nxp.com/pip/BC849B.html

    hFE typ 450 @ Ic = 10uA.

    Graham
     
  11. Guest

    If you look at low Iq LDOs, the bandgap references are using current
    in that neighborhood. Do the math. Figure 10ua for the bandgap, or 5ua
    each side, the say 8:1 for the transistor ratio. So you have ic=5/8uA,
    with transistors that are not really exceptional. Note such circuits
    have replica devices out the wazoo to control (i.e. replicate) leakage
    due to the low beta. The poster never indicated the function of the
    transistor, so it is not known if such tricks are required.
     
  12. Greg Neff

    Greg Neff Guest

    Not enough gain at the low end, but thanks for the suggestion.

    ================================

    Greg Neff
    VP Engineering
    *Microsym* Computers Inc.
     
  13. Greg Neff

    Greg Neff Guest

    BC847C looks promising. Lots of gain at the low end, and at low
    temperatures.

    ================================

    Greg Neff
    VP Engineering
    *Microsym* Computers Inc.
     
  14. Greg Neff

    Greg Neff Guest

    Looks like a good candidate, thanks.

    It would be nice if the web sites were better designed to make it
    easier to track down these parts on parametrics, but that is an
    entirely different topic.

    ================================

    Greg Neff
    VP Engineering
    *Microsym* Computers Inc.
     
  15. Greg Neff

    Greg Neff Guest

    I am not at liberty to discuss details of the circuit, other than I
    need low frequency (DC - 100Hz) linear operation, with Vce around 10V.

    ================================

    Greg Neff
    VP Engineering
    *Microsym* Computers Inc.
     
  16. Greg Neff

    Greg Neff Guest

    A FET would be great, but I don't have enough headroom to drive the
    gate. I can barely accommodate Vbe.

    ================================

    Greg Neff
    VP Engineering
    *Microsym* Computers Inc.
     
  17. Joerg

    Joerg Guest

    Yes, but the beta is often not an important parameter in my designs.
    Cost differences of fractional cents often are.
     
  18. Joerg

    Joerg Guest

    Amen to that! Mfgs should stop hiring script kiddies and talk to actual
    web site users beforehand.
     
  19. Fred Bartoli

    Fred Bartoli Guest

    Joerg a écrit :

    Yep, the 3904 isn't so great, noise-wise.

    Have a look at Zetex (their low Vcesat eline, like FFMT618/718...)
    They're not noise speced but I did measure them at about 0.5nV/rtHz and
    1/f corner under 100Hz.
    Lots of choice in this family.

    Ohooo, listen Graham... they're much better than the 4400.
     
  20. Joerg

    Joerg Guest

    Wow, that would be cool. A bit pricey but if they really hold the line
    under 1nv/rtHz that would be ok. Do you remember at which current you
    had measured them?

    In case others need such a transistor, it's not FFMT but FMMT618. Data
    sheet:
    http://www.zetex.com/3.0/pdf/FMMT619.pdf
     
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