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Discussion in 'Electronic Design' started by Kevin Weddle, Mar 4, 2004.

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  1. Kevin Weddle

    Kevin Weddle Guest

    Does anybody notice the lack of transistor biasing in many circuits.
    Whenever I am able to correctly bias several transistors in a circuit,
    Im' still left with a signal driving a PN junction.
     
  2. Eh??
     
  3. Kevin Weddle wrote...
    I hadn't noticed. :>) There are several simple ways to deal with
    an awkward extra PN-junction voltage, if that's what's bothering you.
    Here's one for an NPN grounded-emitter amplifier stage, add a PNP
    emitter-follower first. The dc bias-voltage presented at the input
    appears across the bias-current-setting emitter resistor, without an
    extra PN junction voltage drop.

    | | | amplified
    | pullup +--- signal out
    | resistor |
    | | |/
    | +---| npn
    | pnp | |\V
    | |/V |
    | ----| bias-set
    | |\ resistor
    | | |
    | gnd gnd

    Another completely different approach is shown in figure 2.41 in
    our book, AoE.

    Thanks,
    - Win

    whill_at_picovolt-dot-com
     
  4. Robert Baer

    Robert Baer Guest

    Furthermore that "trick" will allow the use of a high voltage power
    NPN (which has low current gain) and a small signal PNP which has a low
    voltage rating but high current gain.
    The net result is a larger voltage gain and better thermal stability
    due to Vbe tracking.
     
  5. I read in sci.electronics.design that Kevin Weddle
    I don't think you have described your subject clearly enough to allow a
    useful comment to be made.
     
  6. Jim Thompson

    Jim Thompson Guest

    Developed by Yours Truly some gezillion years ago and can be seen in
    more detail (with analysis) in "HearingAidSlidingClassA.pdf" on the
    S.E.D/Schematics page of my website.

    This scheme was actually first applied to a video amplifier design
    that was developed for Wright-Patterson AFB in the '60's. There was a
    paper presented (IIRC, in the precursor to ISSCC), I'll see if I can
    track it down.

    ...Jim Thompson
    --
    | James E.Thompson, P.E. | mens |
    | Analog Innovations, Inc. | et |
    | Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
    | Phoenix, Arizona Voice:(480)460-2350 | |
    | E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
    | http://www.analog-innovations.com | 1962 |

    Will you still need me, will you still feed me, when I'm sixty-four?
     
  7. Kevin Weddle

    Kevin Weddle Guest

    This is about how the lack of resistance and the lack of source
    voltage in many circuits lends itself to transistors that aren't
    biased. It seems that the PN junction is adequate for signal
    application, but a correctly biased transistor is better.
     
  8. Kevin Weddle

    Kevin Weddle Guest

    I understand how your example works. It's nice that the meaning of the
    voltage is not lost in this case. I would rather not deal with a PN
    junction, but when I do, the end result can still be seen. In other
    words, if you guess about the PN junction and get the correct output,
    then its okay.
     
  9. I read in sci.electronics.design that Kevin Weddle
    Incomprehensible.
     
  10. This is about how the lack of understanding and the lack of logical
    thinking in many people lends itself to postings that aren't
    comprehensible. It seems that the OP's skill is adequate for using a
    newsreader, but a properly educated engineer is better.
     
  11. Agreed. Even if I don't know the answer to a question (not unusual) I
    can still understand it. This guy might as well come from Mars. :-|
     
  12. Actually, the posting APPEARS to be a screwed up translation from
    a non-English language. I guess the statement to claim: "PN junctions
    don't have to be biased, but work better if transistors are properly
    biased".

    That statement seems to come from someone who is isolated
    from anyone who can help them gain a basic understanding of
    typical electronics design issues. That statement APPEARS to
    be an answerback from a misinterpreted explanation.

    I'd like to think that the poster who made the nonsensical statements
    is either very young or has some problems with English usage.

    Hopefully, we'd all try to gain a more tolerant attitude about
    those who are newbies... The truly irritating claims are
    more like 'NPNs are better than PNPs because it is better
    to work with positive supply voltages' or somesuch from someone
    who knows very little and has an overly strong sense of the
    worth of their opinions. (That kind of claim might come from
    someone who does know something about technology, but doesn't
    have any common sense otherwise.)

    Rather than judge the apparently newbie so harshly, it might
    have been kinder to ask about the context of the claims.

    Perhaps the kinder response might have been:

    ===
    PN junction and transistor biasing is done with a purpose. Often,
    when there is zero bias, the semiconductor component is
    actually biased that way for a reason. Most often, when a
    transistor is biased, the operating parameters (even if zero
    quiescent current flow) are carefully considered.

    It would be good to try to learn a little more about both
    the technical use of transistors and a bare minimum about
    the internal component behaviors.
    ===

    My own response isn't complete, and the posts made by the
    poster ALMOST seemed to be poorly translated English...

    John
     
  13. Fred Bloggs

    Fred Bloggs Guest

    The "signal driving a PN junction" part almost certainly means there is
    confusion over quiescent dc, ac, and total instantaneous circuit
    signals- and has nothing to do with residual Vbe's.
     
  14. The OP's name sounded very English to me, so it didn't occur to me that
    the text could be a poor translation from another language. My own
    impression was that the OP just assembled a few technical terms into a
    couple of grammatically correct sentences without a coherent sense. Note
    that others have expressed their confusion in this thread already, and
    that I answered to a followup post by the OP that did nothing to clarify
    the matter. My posting was meant to be a "persiflage". Offense was not
    intended.
     
  15. Fred Bloggs

    Fred Bloggs Guest

    The OP is apparently posting from an asylum through a web-TV roadrunner
    account in the US.
     
  16. Jim Thompson

    Jim Thompson Guest

    I have gone over some of the equations with a pen, re-scanned
    "HearingAidSlidingClassA.pdf", so that it is more readable; and
    re-loaded it to the website.

    BTW, the results shown in AoE, Figure 2.41 are in error... the output
    voltage is NOT +10V. Can anyone provide a correct expression for the
    quiescent operating point?

    ...Jim Thompson
    --
    | James E.Thompson, P.E. | mens |
    | Analog Innovations, Inc. | et |
    | Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
    | Phoenix, Arizona Voice:(480)460-2350 | |
    | E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
    | http://www.analog-innovations.com | 1962 |

    Will you still need me, will you still feed me, when I'm sixty-four?
     
  17. Fred Bloggs

    Fred Bloggs Guest

    The deal was to get equal collector currents and halve the collector
    resistor on the output transistor. The output transistor Vbe2
    corresponds to the compensating transistor *emitter* current of
    (Vcc-Ib1*Rb1-Vbe1)/Rc1-Ib2 so that the emitter current of the output is
    the same making the output collector less by Ib2 or
    (Vcc-Ib1*Rb1-Vbe1)/Rc1-2*Ib2, and therefore the output voltage is
    Vcc-((Vcc-Ib1*Rb1-Vbe1)/Rc1-2*Ib2)*Rc2 then since Rc2/Rc1=0.5 and
    Rb1=Rc2, this reduces to (Vcc+Vbe)/2+(0.5*Ib1+2*Ib2)*Rc2. Yep- looks
    like you're right.
     
  18. Jim Thompson

    Jim Thompson Guest

    Of course I'm right, Fred! And you knew the answer before you even
    put math to the question ;-)

    ...Jim Thompson
    --
    | James E.Thompson, P.E. | mens |
    | Analog Innovations, Inc. | et |
    | Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
    | Phoenix, Arizona Voice:(480)460-2350 | |
    | E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
    | http://www.analog-innovations.com | 1962 |

    Will you still need me, will you still feed me, when I'm sixty-four?
     
  19. Fred Bloggs

    Fred Bloggs Guest


    Please view in a fixed-width font such as Courier.


    Vcc
    ---
    |
    +-------+-------+
    | |
    --- ---
    \ / \ /
    --- ---
    | |
    | |
    / | Ie1=(Vcc-Vdiode-Vbe)/2R
    2R /
    / R
    \ / Ic2~ Ie2=Ie1
    | \
    | |
    +-----+--------+ | Vc2=Vcc-Vdiode-Ic2*R
    | | | |
    / | | |
    R | | | Vc2=Vcc-Vdiode-1/2*(Vcc-Vdiode-Vbe)
    / | | |
    \ c | c =Vcc/2
    | |/ | |/
    +---+ Q1 +----+ Q2
    |\ |\
    e e
    | |
    | |
    +------+--------+
    |
     
  20. Jim Thompson

    Jim Thompson Guest

    [snip]
    You left out a base resistor, but otherwise correct.

    This is SOP in my microchip world.

    ...Jim Thompson
    --
    | James E.Thompson, P.E. | mens |
    | Analog Innovations, Inc. | et |
    | Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
    | Phoenix, Arizona Voice:(480)460-2350 | |
    | E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
    | http://www.analog-innovations.com | 1962 |

    Will you still need me, will you still feed me, when I'm sixty-four?
     
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