Fw: Re: Re: [Amps] 2879 biasing
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.
> > 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
> 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..
> > -
> > [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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