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Are small signal npn transistors really so different from one another?

Discussion in 'Electronic Design' started by Paul Burridge, Dec 20, 2003.

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  1. Hi guys,

    I just designed (in Spice) this chain of three c/e untuned amplifying
    stages to get from 50uV in to 60mV out (at 40Mhz) using what I thought
    were 2N3904 transistors. Optimised the biasing arrangements over
    several hours to get maximum Vgain. Now I've realised I was using the
    model for the 2N5769 instead by mistake. Upon 'plugging back in' the
    2n3904 models into the schematic however, a fresh simulation showed
    the ouput was now only 4uV! I can't believe this substitution would
    result in such a deterioration of the amp's efficiency. I'm
    particularly pissed off as I don't have any 2N5769s lying around to
    use for the actual circuit; I'd been planning on using some of my
    extensive stock of '3904s.
    Can anyone verify that such a huge disparity in gain is indeed
    possilbe? And the quickest way around the problem to get my 60mV back?


  2. If you are saying that one type produces an output of 60 mv (net gain
    of 1200, which is pretty great at 40 MHz) and the other produces an
    output of 4 uv (net gain of .08), then I agree that this is very
    unexpected. They are not that different (one has a higher collector
    voltage rating).

    Unfortunately, there is almost no high frequency spec, on the data
    sheets that are done under the same conditions, to allow an easy
    comparison between them.

    The data sheet for the 5769 is very terse, but this one from the same
    process family has more details:

    I think you made some inadvertent change in the schematic when you
    swapped transistors.
  3. Look at the switching times...
    It tells the story CLEARLY.

    | __O Thomas C. Sefranek
    |_-\<,_ Amateur Radio Operator: WA1RHP
    (*)/ (*) Bicycle mobile on 145.41, 448.625 MHz
  4. Gary Lecomte

    Gary Lecomte Guest

    While small signal transistors are usually simular, they don't make
    Thousands of different ones, Just for the fun of it. They do have
    different currents, Frequency ranges, voltages, HFE's, gains and other

    But if you need to incease the gain of the 2N3904, connecting two
    together in a "Darlington configuration" should do it for you.

    However also remember different transistors require different base
    bias currents.

    Take care.....Gary
  5. Paul Burridge wrote...
    Paul, good circuit designs should not be dependent upon highly-specific
    properties of individual components, unless those properties come from
    basic device physics, e.g. Ebers-Moll, or other reliable considerations.

    For example, the dc operating bias points of amplifier circuits should
    not be dependent upon a transistor's beta, or the exact value of Vbe.

    Unfortunately optimizing a design in Spice can lead exactly to such a
    bad result, wherein the design will only work with said Spice device,
    at a specific temperature and supply voltage. I wonder if you have
    fallen into such a trap, so that one of your stages is saturated, or
    otherwise rendered non-operational by changing transistors. Please
    note that in real life such a design would also fail when made with
    the normal part-to-part variation of the same type. BTW, I note the
    two Spice models you posted are actually quite similar transistors,
    and a good design should operate similarly with either type. One has
    more capacitance, etc, so the bandwidth or HF gain could suffer some.

    - Win

  6. Fred Bloggs

    Fred Bloggs Guest

    Chances are that your optimized biasing is putting something in cut-off
    or saturation. You might run a DC Operating Point analysis and see
    what's up.
  7. R.Legg

    R.Legg Guest

    It won't.

    Troubleshoot and copare the models the same way you would a 'real'
    circuit, section by section.

    Don't forget DC static bias.

  8. What has switching times to do with it? I'm not using these devices in
    a switching application; rather an RF amplifier.
  9. Thanks all,

    I've run two operating point checks; one for each circuit where in
    each case all the transistors are either one type or the other. Got
    some very interesting results...

    Remember, 3 stages in c/e mode and the voltages around each transistor
    were as follows:

    Circuit using the 2N5769s:

    Q1: e=4.39; b=5.04; c=4.79
    Q2: e=4.46; b=5.12; c=7.60
    Q3: e=4.22; b=4.89; c=5.06

    Circuit using the 2N3904s:

    Q1: e=4.50; b=5.19; c=4.59
    Q2: e=5.07: b=5.76; c=6.95
    Q3: e=4.49; b=5.20; c=4.59

    As you can see, there are some very low Vce readings. I guess this
    means I'm too close to saturation in some instances?
    Comments/observations gentlemen, please.
  10. Leon Heller

    Leon Heller Guest

    I think your problem is due to 'designing' with SPICE. You'd be better
    off using proper design techniques then using SPICE to check your design.

  11. Paul Burridge wrote...
    Indeed. In general for high-frequency amplifiers you want
    to use Vce = 2V or more to reduce Ccb. Furthermore the
    last stage needs more room for output-voltage swing.

    - Win

  12. I am amazed that you got much gain out of either of them Do these two
    versions still show the 2000:1 gain variation you spoke of, earlier?

    Regardless, I am almost certain that you are not anywhere optimum
    biasing in either case.

    Would you post your schematic on a.b.s.e?
  13. Jim Thompson

    Jim Thompson Guest


    There are four BASIC factors that differentiate bipolar transistors...

    (1) Physical area... defines maximum current AND power
    (2) Doping levels... maximum voltage
    (3) Finger structures... current handling AND fT
    (4) Packaging... power handling and high frequency capability

    Beta, which most everyone has a fetish over, is a function of the
    first three.

    ...Jim Thompson
  14. Jim Thompson

    Jim Thompson Guest

    I'd like to see the schematic also.

    ...Jim Thompson
  15. John Popelish wrote...
    It's only a gain of about 10 per stage on average, and Paul
    didn't specify the output load. Maybe it's not 50 ohms.

    - Win

  16. You're absolutely right as ever. I was forgetting this fundamental
    principle. :-(
    Yup. I've been such a dumb-ass as you say in America. Maybe
    transistors give best gain when driven close to saturation, I don't
    know, but it would certainly account for how my optimising the biasing
    has rendered a model change a disaster.
    I've been *very* slapdash since I discovered Spice. Instead of sitting
    down and working out the bias conditions properly on paper first, I
    typically draw up a schematic on the computer and pull values straight
    out of the air as they 'look about right under the circumstances.' I
    then wonder why the thing doesn't work very well (or at all). Only on
    going back to doing the sums on paper can I see where I've gone wrong
    - and it's often by an order of magnitude with currents and voltages
    all over the place. It's laziness, I guess, and it doesn't work in the
    long run! The temptation is it's just *so* easy to try a dozen
    different combination of values in minutes just to eek out a bit more
    gain here and there. But I now know that's not a very good idea!
  17. For a broadband amplifier at 40 MHz built by trial and error, I am
    still amazed at an average stage voltage gain of 10, unless he got it
    by finishing with an output stage that drives no external load.
  18. It's 380 ohms in this particular case, chaps.
  19. I'll try to do so later, but am off up the pub in a minute. Does
    anyone know if it's possible to post an LTSpice schematic to a
    newsgroup in a way that's legible to everyone?
  20. Guilty. See my other reply to Win.
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