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[Amps] 2879 biasing

To: <amps@contesting.com>
Subject: [Amps] 2879 biasing
From: kd6tas at netzero.net (Bill Eitner)
Date: Sat Feb 22 19:16:24 2003
> Bill, 
> Have you ever measured the Zero Signal Ic with temp 
> changes Bill..? 

    Sure.  Hasn't everyone?
    Verification is as simple as heating a rectifier
    diode with a soldering iron while watching the 
    resistance across it.  More heat--less resistance.
    When applied to a 2SC2879 transistor, more 
    heat means more B-E current which leads to
    more C-E current.  A good transistor, a power
    supply, a resistor, and a soldering iron is all you
    need to verify it directly.  I don't expect you to
    believe me.  I fully realize that you don't know
    anything about me or what I've done.  I've known
    about this forum for quite awhile, but have only
    recently subscribed to it.  I fully expect to have
    to prove everything I say here.  I don't have a
    problem with that.

> Or have any data/examples to reference..?

    Look at Application Note EB-63 in the Motorola
    RF Device Data Book.  That's arguably the design
    that got the ball rolling as far as that technology 
    is concerned.  You'll notice that the bias circuit 
    has a diode that is thermally connected to the
    heatsink.  The bias voltage is equal to the voltage
    drop across the diode--which decreases with heat.
    The idea was that the diode drop, and therefore the
    bias, would stay in step with the drop across the
    RF power transistor B-E junctions therefore maintaining
    the operating class.

    Application Note AN-762 builds upon the EB-63
    design and is applicable to sideband.  Its bias circuit
    employs a regulator to deal with supply variations, but
    it's still based on the drop across a transistor junction
    that is thermally associated with the RF power transistor
    flanges.

> Transition from "a little dirty" to "clean" with temp seems 
> a bit much. 

    The increase in idle current with increases in temperature,
    and therefore the movement in operating class toward Class
    A, will depend on flange temperature regulation and bias circuit
    design.  As you progress through this you'll see that those 
    variables don't have standardized designs associated with them.

    I stand firmly behind the quote below.  The use of undersized
    heatsinks with no complimentary forced-air assistance, and 
    oversimplified (a nice way of saying misguided) bias circuits
    that overbias the transistors when they are at room temperature,
    are the rule rather than the exception when it comes to homebrew
    amps because those variables can be neglected that much and
    the amp will still work for awhile. In the quote below I'm
    assuming that the rest of the amp is properly designed.  That
    means enough degenerative feedback and isolation between
    the input and output signals that the amp is stable regardless
    of operating class (which is rarely the case).  When that's the
    case the spectral purity will improve as the amp's operating
    class moves from Class B to Class AB.  In most cases the
    distortion and related spectral anomolies change from one
    type to another.  The change in operating class decreases
    the crossover distortion and related in-band spectral impurities
    while simultaneously increasing the small signal gain which
    increases the harmonic content.  The good news is that the
    latter is easier to deal with through the use of filters, but there's
    a limit.  In my opinion, it's best not to tempt fate by overbiasing
    the amp in the first place.
--
> skipp
> -
> [snip]
> >>>>When adjusting the bias for a 2sc2879 amp operating 
> >>>>at 15v, what
> -    
> Remember that unless you're using a temperature
>     compensating bias circuit the resting collector current
>     will rise as the heatsink, flange, and B-E junction(s)
>     increase in temperature.  In a nutshell, what is Class B
>     and a little dirty at room temperature can quickly become
>     Class AB and clean after the first QSO--especially with
>     15 volts on the collector(s).
> --
> -------------------------------------------------------------------
>  -=[Bill Eitner]=-


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