Manfred,
Can you explain what this design is trying to achieve with separate
transformers for the output (T1) and for supplying DC (T2) ? Why not
the more traditional center-tapped transformer that acts as the output
transformer and the method of suppling DC ?
73,
Steve, N2IC
On 04/23/2017 02:07 PM, Manfred Mornhinweg wrote:
http://www.kg9e.net/projects/hfpacker/
It may be telling that K5OOR has resigned the amp since my kit was
built. The current one being sold appears to be physically larger than
mine.
Documentation:
https://storage.googleapis.com/wzukusers/user-17403798/documents/58c313c4618a4lexQCfw/Construction%20Manual%20miniHFPA%20with%20schematics%20and%20BOM%202017%2003%2010.pdf
The required tight drain-to-drain coupling can either be implemented via
a center-tapped output transformer, but it MUST be a real, magnetically
coupled center tap, _not_ a half-turn tap; or a bifiliar feed choke must
be used. This amplifier uses the latter option. But unfortunately the
bifilar choke used isn't up to the task, and judging from the published
text, the designer hasn't fully understood the requirements for this choke.
The choke he used has 10 bifiliar turns on 2 stacked FT-50-43 cores.
This must give roughly 100µH per side, which is FAR more inductance than
needed. This much inductance wouldn't really hurt here, but the problem
is that along with the high inductance comes a significant leakage
inductance. And the leakage inductance acts like two independent
(uncoupled) chokes in series with the (coupled) bifiliar choke,
destroying the coupling between the FET drains if this leakage
inductance is too high.
I would have to build that bifiliar choke to measure the actual leakage
inductance, and I'm too lazy now to do that. But judging from
experience, I would expect around 0.5µH leakage inductance per side.
That's an impedance from 6 ohm on 160 meters, up to 91 ohm on 10 meters.
Meanwhile the drain load impedance in this amplifier is 11 ohm per side.
So the coupling between drains is modest on 160m, poor on 80m, and gets
worse on higher bands, to the point that from 30m upwards there is
essentially no drain-to-drain coupling. And this is the fundamental
explanation for the poor efficiency of this amplifier! The bifiliar
choke would need to be re-designed so that it produces tight coupling
between the two drains, all the way into VHF, to cover at least the
lower harmonics of 10m. At 11 ohm drain impedance this can be done,
although it's not really easy. Instead with a high power LDMOSFET amp,
or a 100W 12V powered amp, both of which have drain load impedances
around 1 ohm, it cannot be done, to the best of my present knowledge -
and that's the explanation for the "typical" 45% efficiency of most HF
push-pull broadband amps we see!
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