Fw: Re: Re: [Amps] 2879 biasing

skipp isaham nospam4me at juno.com
Sun Feb 23 20:20:42 EST 2003

Just as I had expected...

--------- Forwarded message ----------
From: "Bill Eitner" <kd6tas at netzero.net>
    Good luck with your trolling.
----- Original Message ----- 
From: "skipp s isaham" <nospam4me at juno.com>
To: <amps at contesting.com>
Sent: February 22, 2003 8:43 PM
Subject: Fw: Re: [Amps] 2879 biasing

> > Bill, 
> > Have you ever measured the Zero Signal Ic with temp 
> > changes Bill..? 
> -
> - >   Sure.  Hasn't everyone?
> -
> No, and I was wondering if you had any actual 
> hard value data that YOU measured and could share?
> Not something from a data or text book?  
> -
>     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.
> -
> zzzzzzzzzzzzzzzzz...........................!!!!!
> -
> > Or have any data/examples to reference..?
> -
>     Look at Application Note EB-63 in the Motorola
>     RF Device Data Book.  
> - 
> I'm talking about actual values that you've measured 
> or set?  Are you talking from a text/data book..? and/or 
> do you have any actual values from your projects/ 
> work and how were those values determined?
> - 
> -
>    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
>     heat sink.  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.
> -
> Easily quoted from data/theory books...  I'm looking 
> actual values you've had experience with. Real stuff... 
> -
>     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.
> -
> Thanks, I have all the Data Books and app notes. My 
> doubts bring me to again state the just below 
> and ask if you have any actual working circuit data..? 
> -
> > 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.
> - 
> You post a batch of very generic fodder.  Other than quoting 
> book 101, how about some actual examples you've done. 
> Feel free to use the 2879 device if you actually have any 
> true data to support your "little dirty to clean" statement? 
> -
>     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
>     a limit.  In my opinion, it's best not to tempt fate by overbiasing
>     the amp in the first place.
> --
> You post common rf (and long winded) data/text book 101.  How 
> about something real based on the 2879..?  ... or any other 
> device that you've worked with..?   I don't care to read your 
> book theory 101 on bias and bias tracking circuits.  I want 
> some data on a working circuit and how you got it..?  How 
> do you know the amplifier becomes "clean" as it heats 
> up.  Why would this assumption be good or bad news..?
> -
> Otherwise... got milk..?
> your turn..
> -
>  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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