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Weird transistor biasing?

Discussion in 'Electronic Design' started by Jacobe Hazzard, Dec 6, 2003.

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  1. I am looking at this schematic:
    http://www.geocities.com/j4_student/NewWayWah.gif and scratching my head. Is
    there an error in the buffer section?

    A buffer is very desirable for the circuit, because it economizes on
    bypassing with a SPDT instead of a DPDT, and therefore the circuit is
    connected even when not in use. BUT

    How is Q3 biased? Looks like it's set up as an emmiter follower, but with a
    1.8M collector load? And it also looks like the base is sitting at ground,
    and will do terrible things to the ac-coupled input. Am I missing something
    or is this schematic very messed?

    Thanks,
    Adam
     
  2. OK crisis averted. I found another schematic with the same component values
    but in a configuration that makes sense to me:
    http://www.geocities.com/j4_student/crybaby95.gif

    Although the biasing is still very poor, and will depend on the gain of the
    transistor (so will probably drift, am I right?)

    jacobe
     
  3. Robert Baer

    Robert Baer Guest

    Reference does not work, i get:
    We're sorry, but this page is currently unavailable for viewing.

    Not seeing the schematic does not help, but a common base useage is
    not unknown.
    One use is for low Z RF input as a signal amplifier; it does not seem
    that is what you have, but it is possible.
    Another use is as a current follower and/or level translator; hard to
    say that is what you have, but it is possible.
    Yet another use is as a cascode amplifier (stacked on top of another
    transistor amplifier), usually for RF but also good in audio and other
    applications requiring high gain; it does not seem that is what you
    have.
     
  4. Robert Baer

    Robert Baer Guest

    Same access problem as the other reference.
    Can make no meaningful comment.
     
  5. Both are still working for me, as posted. I cleared the cache and tried
    again with no problems, but you could follow the links to "Muti-Crybaby" and
    "Crybaby GCB-95 Wah" under the section "Wahs"
    from http://www.geocities.com/j4_student/schematics.htm.

    I don't think the biasing is any of the possibilities you mention, I think
    it's just wrong.
     
  6. Based on a very quick glance, I see the same problems you do. Don't see how
    that emitter follower is going to get biased.
     
  7. Ban

    Ban Guest

    Jacobe Hazzard wrote:
    || ||| Jacobe Hazzard wrote:
    ||||
    |||| OK crisis averted. I found another schematic with the
    |||| same component values but in a configuration that makes
    |||| sense to me:
    |||| http://www.geocities.com/j4_student/crybaby95.gif
    ||||
    |||| Although the biasing is still very poor, and will
    |||| depend on the gain of the transistor (so will probably
    |||| drift, am I right?)
    ||||
    |||| jacobe
    |||
    ||| Same access problem as the other reference.
    ||| Can make no meaningful comment.
    ||
    || Both are still working for me, as posted. I cleared the cache and
    || tried again with no problems, but you could follow the links to
    || "Muti-Crybaby" and "Crybaby GCB-95 Wah" under the section "Wahs"
    || from http://www.geocities.com/j4_student/schematics.htm.
    ||
    || I don't think the biasing is any of the possibilities you mention, I
    || think it's just wrong.

    The first one should have R14 to the base and Collector to +U.
    The second looks good to me.
     
  8. Baphomet

    Baphomet Guest

    Probably, the 1.8 meg resistor was supposed to go to the base and the
    collector directly to the + supply.
     
  9. Baphomet wrote...
    I agree. And the circuit should have an emitter follower output.

    Thanks,
    - Win

    whill_at_picovolt-dot-com
     
  10. The circuit is assuming that it's looking into a guitar-amp-like impedance,
    probably around half a meg. Else that 1k in series with the battery
    wouldn't be a very good idea.
     
  11. Walter Harley wrote...
    Yes of course. But the emitter follower is to reduce the source
    impedance of the signal traveling on an external cable, to help
    insure it won't be susceptible to noise pickup, to allow for long
    cables with high capacitance, reduce RFI sensitivity, etc., etc.

    Thanks,
    - Win

    whill_at_picovolt-dot-com
     
  12. No, the emitter follower is to increase the input impedance seen by the
    driving circuit or instrument. With guitars you are not only concerned with
    attenuation due to loading but damping of the strings due to current flow in
    the pickups. This is known as 'tone suck' and happens even when this pedal
    is set to 'bypass'.
     
  13. James Meyer

    James Meyer Guest

    I didn't look at the first site but the one referenced above shows a
    normal emitter follower configuration with a darlington transistor (beta of
    aprox. 1,000), a 1.8 Meg biasing resistor to the base, the collector connected
    to the (9 volt) battery, and a 10K emitter resistor.

    If we assume about .6 miliamps of emitter current, the voltage across
    the 10K resistor will be about 6 volts. Adding about 1.4 volts for the B-E
    junction leaves about 1.6 volts across the 1.8 meg resistor for a biasing
    current of about .8 microamps. Multiply that by the beta for the transistor and
    it confirms our original guess about the emitter current.

    All in all, it looks to me like a perfectly normal emitter follower with
    no problems at all.

    Jim
     
  14. Jacobe Hazzard wrote...
    Sounds like you're referring to an emitter follower input.
    Maybe one could be useful, maybe not, given Meg-ohm bias
    resistors. But I was referring to the lack of an emitter-
    follower output to reduce the output impedance and thereby
    yield the benefits I mentioned. (Note, it's doubtful the
    destination amplifier, etc., has a low Zin, so the effect
    of increasing the apparent load impedance isn't needed.)
    Emitter follower Zout = Zin/beta + 25/Ic ohms, where Ic is
    the transistor bias current (see AoE pages 65-72 and 81-82).
    One can easily get Zout under 10 to 20 ohms, which is very
    useful to insure a robust signal transfer.

    Thanks,
    - Win

    whill_at_picovolt-dot-com
     
  15. How do you come to that assumption for emitter current?
    The beta would have to be lower that 1,000 for your stated numbers to work.
    And again, if beta drifts (between transistors, with time, temperature), so
    does biasing.
     
  16. message
    The way I see it the base is biased by the voltage divider formed by 1.8M
    and hfe*10k, so is unstable.
     
  17. James Meyer

    James Meyer Guest

    im
    Trial and error. I picked an emitter current and worked backwards from
    there. When the numbers didn't work out correctly, I changed my original guess.
    When the numbers got close, I stopped changing them.
    The figures are close. I could have refined them again to get closer,
    but I stopped when I determined that the circuit should work as it was drawn.
    Biasing of an emitter follower is extremely forgiving since its voltage
    gain is only one. In the illustrated case, a change of plus or minus 100
    percent in either transistor beta or bias current would still result in a fully
    functional stage. Bias is more critical for high gain voltage amplifiers.

    Jim
     
  18. James Meyer

    James Meyer Guest

    That's your opinion. You've seen my opinion.

    In the circuit we're looking at, the transistor is biased with a base
    current and the voltage at the base is only incidental to to the bias and not
    the primary bias source.

    Jim
     
  19. Ban

    Ban Guest

    Jacobe Hazzard wrote:
    ||||| http://www.geocities.com/j4_student/crybaby95.gif
    |||||
    ||
    || The way I see it the base is biased by the voltage divider formed by
    || 1.8M and hfe*10k, so is unstable.

    Jacobe,
    I hadn't seen in my first mail that this was a darlington. this would not
    make the circuit unstable, but the base voltage would be pretty much on the
    supply rail. So an assymetrical limiting will occur with input levels above
    a few 10s of millivolts.
    But with a normal transistor with a hfe of 30...100 the biasing looks
    normal. I just put that into my simulator and get with a 2N2222 transistor
    some reasonable 4V dc on the emitter. The input current would be 1.12uA with
    1Veff, so around 880k input impedance. So probably the guy who drew this
    schematic messed up too.
     
  20. Robert Baer

    Robert Baer Guest

    The first, "Original Crybaby", i see nothing strange. The 2nd NPN is
    used as an emitter follower where the base drive comes from both the
    collector of the first, as well as higher frequencies coupled to the
    base can be enhanced by the pot.
    The "multi-crybaby" and the "95" are the same in structure, just refer
    to the 3rd transistor.
    There is no problem in biasing.
     
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