----- Original Message -----
From: "Jim Thomson" <jim.thom@telus.net>
To: <amps@contesting.com>
Sent: Saturday, June 10, 2017 1:48 AM
Subject: [Amps] LDMOS availability
Date: Fri, 9 Jun 2017 22:02:53 -0600
From: Jim Garland <4cx250b@miamioh.edu>
To: amps group <amps@contesting.com>
Subject: Re: [Amps] LDMOS availability
<There are two issues, here, one being the definition of Class C, and the
<other being linearity. I'm inclined to agree with Bill on both. It's not
hard to imagine modulating the bias, so that on voice peaks, for
example, the amp moves in the direction of Class B, but regresses to
Class C at low drive levels.In that case, the class of operation varies
with the drive. That said, Class C is by definition non-and that occurs
whenever the device doesn't conduct over a full RF cycle.
I have a homebrew amp with two-tiered bias. On SSB, in pauses in speech,
the amp is biased at a very low resting current (approaching Class C) to
cut down on heat, but on voice peaks where the linearity is needed, it
switches to Class B. One has to be careful playing that trick, however,
to avoid switching transients.
73,
Jim W8ZR
## Another amp config that switches about, is the infamous G2DAF circuit.
I have a pair of 4-400s in that config. Done right, they operate just
fine, but the
component values have to be chosen very very carefully..esp the screen
caps. Nobody
has ever complained of imd...and cant tell the difference between the
G2DAF amp, and the
GG amps..when I toggle between them...while listening off freq. A lot of
G2DAF amps are not
configured correctly, and I dont recommend the design for any new
construction projects.
I have seen the G2DAF config used on 4-400, 4-1000, 4CX-1000, 4CX-250B.
They essentially switch from class C-B-AB..and back again from AB-B-C.
## I looked into the two-tiered bias scheme back in the 70s. When it 1st
came out
in QST, its function was to cut off idle current between dots and dashes
for qsk cw.
## To avoid the dreaded switching transients, esp on ssb, the
three-tiered bias scheme
was introduced. Cut off on RX, the bias V is high...like 40-60 vdc.
With PTT activated,
the bias V is reduced, like 15-20 vdc, so idle current is real low, like
20-40 ma. With RF
detected, bias V toggles to its normal linear state, like 5-8 vdc, such
that normal idle current occurs.
While talking on ssb, then the bias is just toggling between say 15-20
vdc...and 5-8 vdc.
With the two-tiered scheme, the bias is toggling between say 40 - 60
vdc and 5-8 vdc.
That takes time..and the amp ends up being non linear while the drive
level increases..esp
at low levels. If you have any amount of back ground noise, their will be
enough RF from the xcvr,
even with just a few milliwatts, to toggle the bias, defeating the
purpose of the bi or ...tri-state bias scheme.
A simple noise gate..or downward expander, would solve that issue. A noise
gate is just a high ratio
downward expander.
## Even with the three – tiered scheme, its still not fast enough to
toggle between syllables, so normal
say 5-8 vdc bias stays on for the duration of the word phrases. It will
however, toggle between words..
sometimes.
## I played with it..and finally gave up. Where it sorta works good
is if using PTT-Footswitch SSB.
For VOX ssb, it does not buy you anything..... except between the last
word spoken....and when the vox drops out.
Which is of such short duration, with the typ 300-500 msec vox delay.
### For CW use, I found that its just as easy to simply increase the
bias Voltage a bunch, such that idle current
is really low, like 15-40 ma. Then you get the heat reduction between
dots + dashes. Idle power is WAY down, heat
is WAY down. Tank eff increases a little bit, 1-3 %, further reducing
dissipation when driven full bore. On any GG
amp, you cant cut off the idle current, and run it into class B or Class
C.....other wise you will get key clix. It has to
draw a little bit of idle current, hence the 15-40 ma. 15-40 ma is a
helluva lot better vs 150-220 ma, heat wise.
14-40 ma also works good on FM.
Jim VE7RF
** CQ ran an article around 1949-50 using a sliding bias scheme on a 304TL
which ran gobs of idle current in AB. IIRC it was only partially successfull
at well less than full bore. Full details escape me and I dont have time to
dig in to the library.
Carl
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