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High gain using a push-pull

Discussion in 'Electronic Design' started by Kevin Weddle, Mar 6, 2004.

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  1. Kevin Weddle

    Kevin Weddle Guest

    I am trying to design a push-pull for high gain. How high of gain can
    I expect? I just want a general idea using two transistors. Also, I
    can see that I will have a change in beta and a change in collector
    resistance.
     
  2. Kevin Weddle wrote...
    Before you attempt to design, learn some of the basics about
    bipolar transistors and amplifiers. You could do worse than
    starting with Chapter two of AoE. For example, you'll learn
    that a well-designed amplifier's properties are independant of
    beta (because beta isn't predictable). Instead its open-loop
    gain is dependent on g_m (or equivalently 1/r_e) times the
    collector load resistance. Furthermore, this load resistance
    is generally much smaller than the BJT's intrinsic collector
    resistance, which can be therefore be ignored (assuming any
    common transistor with high Early voltage). Moreover, the
    load resistance is often configured to be independent of the
    actual external load, to insure predictable gain. Finally,
    the use of local or overall feedback can over-ride all else.

    To simplify, a common-emitter transistor amplifier stage can
    be considered to be a form of impedance transformer, which
    provides voltage gain as a function of the impedance ratios.
    The gain for a BJT stage is G = g_m * load R. As we point out
    in Chapter two, gm and load resistance scale with current, and
    if the load resistance is chosen so the collector sits at half
    the supply voltage, as is conventional, the maximum stage gain
    = 20*Vs. With a 200V supply, the maximum gain would be 4,000.

    But this is all pretty silly, because real amplifier circuits
    have various criteria other than maximum gain, and are rarely
    limited to one or two transistors, given their cost of only a
    few pennies. For example, adding a current-source transistor
    to the amplifier can boost the single-stage gain towards 1E6.
    Furthermore, single stage high-gain transistor amplifiers have
    *extremely* high distortion, because g_m varies with current
    (see AoE page 83), and real amplifier designers of course use
    various basic design tricks to avoid this problem, as we show.

    Thanks,
    - Win

    whill_at_picovolt-dot-com
     
  3. I read in sci.electronics.design that Winfield Hill
    Why do I suspect that you are wasting your time? (;_)
     
  4. Bill Sloman

    Bill Sloman Guest

    You probably need to search on the "Early effect" - the voltage of the
    collector of a bipolar transistor effects the base-emitter voltage and
    the current gain. Putting a pair of transistors in cascode can help.
     
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