Connect with us

sot jfet

Discussion in 'Electronic Design' started by John Larkin, Jan 15, 2005.

  1. John Larkin

    John Larkin Guest

    Hi,

    I need a p-channel jfet in a sot-23. Who makes stuff like that?

    John
     
  2. Tim Wescott

    Tim Wescott Guest

    Try ON, Zetex and Philips for starters. Then let us know if you find more.
     
  3. Ken Smith

    Ken Smith Guest

    IIRC Central Semi does too.
     
  4. John Larkin

    John Larkin Guest


    Thanks, guys, but false alarm.

    I've got this box full of programmable time delays. Each one is a PNP
    transistor current source driving a capacitor, reset by a gaasfet,
    making about a 60 ns linear ramp into a comparator with a 16-bit DAC
    on the other side, so I get 0-60 ns delay in about 1 ps steps. The
    current source is a BCX71 PNP inside an opamp feedback loop (fb from
    source resistor off +15 rail), conventional stuff. Like an idiot, I
    left out a series base resistor so the PNP likes to oscillate at
    around 100 MHz, causing a bunch of jitter in the delays. These are
    very dense, rather pretty boards and I'd like to find a fix that's not
    an ugly kluge. A jfet would work, but the SOT-23 jfet pinouts don't
    match up with the PNP.

    So the choices are either a small p-ch mosfet, if I can find one with
    low enough capacitance, or one of those "digital transistors" with a
    built-in base resistor. Or a really slow PNP, if anybody still makes
    one.

    So listen up, kids: ALL EMITTER FOLLOWERS NEED BASE RESISTORS unless I
    give you permission otherwise.

    John
     
  5. John Larkin

    John Larkin Guest

    Thanks, but no. The graphs show Ft as around 300 MHz in the 5-10 mA
    range, almost the same as the BCX71. It will probably oscillate too.
    This is for a $6 billion laser, so I can't take chances.

    I had thought that the output impedance of the opamp would be enough
    base degen to prevent oscillation, but I guess the wire bonds, esd
    diodes, and the output transistors can form a high-enough-Q circuit to
    sustain oscillation. Obviously so, in retrospect.

    If I ask the production folks to lift the base lead and stick an 0402
    resistor in there, they'll be after me with torches and pitchforks.

    John
     
  6. It occurs to me that the transistor may be feeding the oscillations
    because it is too slow to maintain the current steady, but feeds back
    collector voltage to the base. Have you tried a much faster
    transistor lower gain, lower collector to base capacitance, but higher
    fT)?

    Something like:
    http://rocky.digikey.com/WebLib/Panasonic/Web data/2SA1738.pdf
     
  7. Nico Coesel

    Nico Coesel Guest

    The feedback loop itself is stable?
     
  8. Ken Smith

    Ken Smith Guest

    The output impedance of an op-amp is alway such that it makes oscillation
    more likely. They tend to look like parallel LC circuits. The L part is
    because as the frequency increases, the servo loop gain is dropping
    causing a rising impedance. The C part is the various stray capacitances.


    Can you deQ the emitter side of things or slip a bead in the collector's
    path?
     
  9. John Larkin

    John Larkin Guest


    I used the BCX71 because in the past the RF-types (NEC, or the Moto
    parts when they still made them) were *really* bad about oscillating.

    My first experience with this problem was a long time ago, when I
    needed a TTL powerup reset signal. I did this...


    +---------+--------- +5
    | |
    r |
    | c
    +--------b 2N2219
    | e
    | |
    c |
    | +----------ttl gate------> reset sig
    gnd |
    r
    |
    gnd


    which never made a high to the ttl gate, it was oscillating so hard at
    100 MHz or so. A small base resistor, 33 or 47 maybe, generally fixes
    it.

    So most small-signal bipolars are hazardous when operating as emitter
    followers when the base sees a low-Z source. As the transistor gets
    faster, things seem to get worse, with the traces and wirebonds to the
    base becoming microwave resonators.

    Considering how often this seems to happen, at least to me, it's
    surprising how seldom it's mentioned. The AoE example (p 300) is
    similar, but blames the oscillation here on impedance in the collector
    circuit.

    John
     
  10. John Larkin

    John Larkin Guest


    Yes. The opamp Ft is only about 3 MHz, and the oscillation is around
    100. It's definitely local to the transistor.

    John
     
  11. John Larkin

    John Larkin Guest


    The emitter is a 500 ohm resistor, so it's low-Q already. And there's
    already a ferrite in the collector! Base resistance is the solid fix;
    I just got sloppy and left it out.

    So it's a mosfet or a digital transistor, and a pilgrimage to a Holy
    Shrine for penance; Fry's maybe.

    John
     
  12. I read in sci.electronics.design that John Popelish <>
    Inductance in the emitter lead creates a negative impedance at the base.
    I expect that's why the thing oscillates. Inductance in the collector
    lead could possibly also cause trouble, but the oscillation is often
    less persistent.
     
  13. Terry Given

    Terry Given Guest

    I cant prove it without doing the maths, but I suspect an analysis
    similar to that given in the Siliconix MOSPOWER app handbook will show
    emitter followers to be inherently unstable, just like FETs, when all
    the parasitics are taken into account. If you havent read the paper, its
    a good one - rather than evil number crunching a Routh-Hurwitz stability
    analysis is performed. very nice.

    Cheers
    Terry
     
  14. That's not the only problem with this circuit. You're not supposed to
    drive TTL with a current source and pulldown. You're supposed to use an
    NPN sink, and a pullup. They also make reset generator chips, but it's a
    little late for that now. I'd maybe add an NPN where you have your "ttl",
    and some positive feedback to get a good snap-action.

    Cheers!
    Rich
     
  15. John Larkin

    John Larkin Guest

    Who passed that law? I'll drive TTL with whatever makes good logic
    levels.
    Not when I did this. They came along about 20 years later.


    John
     
  16. Guest

    John Larkin wrote:
    (snip)
    To be fair, that example had a d'Arsonval meter movement in the
    collector lead.
     
  17. Rich Grise

    Rich Grise Guest

    Win Hill, of course. ;-)

    Cheers!
    Rich
     
  18. Ken Smith

    Ken Smith Guest

    I cant prove it without doing the maths, but I suspect an analysis
    similar to that given in the Siliconix MOSPOWER app handbook will show
    emitter followers to be inherently unstable,[/QUOTE]

    Perhaps more like "really want to be unstable". You have to connect any
    of a large number of combinations of stray Ls and Cs to make them
    oscillate.


    Consider:

    Vcc
    !
    +--
    ! !
    !/ ---
    ---[L]---+---! --- C2
    ! !\e !
    --- ! !
    --- +--+----- to load
    C1 ! !
    -----+
    !

    This is a Colpitts common collector oscillator. If the Cbe and Cce make
    workable values, the transistor will oscillate with just the inductor.
    With fast transistors not much trace inductance is needed.

    or:

    Vcc
    !
    [L]
    +--
    ! !
    !/ ---
    ---------+---! --- C2
    ! !\e !
    --- ! !
    --- +--+----- to load
    C1 ! !
    -----+
    !

    This is a Colpitts common base oscillator. If the Cbe and Cce make
    workable values, the transistor will oscillate with just the inductor.
    With fast transistors not much trace inductance is needed.
     
  19. Terry Given

    Terry Given Guest


    Perhaps more like "really want to be unstable". You have to connect any
    of a large number of combinations of stray Ls and Cs to make them
    oscillate.


    Consider:

    Vcc
    !
    +--
    ! !
    !/ ---
    ---[L]---+---! --- C2
    ! !\e !
    --- ! !
    --- +--+----- to load
    C1 ! !
    -----+
    !

    This is a Colpitts common collector oscillator. If the Cbe and Cce make
    workable values, the transistor will oscillate with just the inductor.
    With fast transistors not much trace inductance is needed.

    or:

    Vcc
    !
    [L]
    +--
    ! !
    !/ ---
    ---------+---! --- C2
    ! !\e !
    --- ! !
    --- +--+----- to load
    C1 ! !
    -----+
    !

    This is a Colpitts common base oscillator. If the Cbe and Cce make
    workable values, the transistor will oscillate with just the inductor.
    With fast transistors not much trace inductance is needed.
    [/QUOTE]

    Indeed, and nicely put. If you look closely enough, there is an array of
    L's and C's inside every transistor, along with gain. Hence the old ROT
    "R's in 2 of 3 leads"

    Cheers
    Terry
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-