[Amps] Low pass filters and saturated operation
Manfred Mornhinweg
manfred at ludens.cl
Mon Jan 1 09:39:25 EST 2018
Hi all,
after my rather pointless post moments ago about the ICAS-CCS debate,
here is a somewhat more meaty year-starter for those of you who do enjoy
hands-on amplifier building.
I'm currently developing an SDR that uses an MRF1K50N in the final
stage. My intention is to run this in Envelope Tracking mode (ET), with
a linear class AB driver stage and running the final stage in saturated
class-AB. I intend to use a bank of relay-switched series-input low-pass
filters, because such filters offer a high impedance to harmonics. The
LDMOSFET, running in saturation, behaves closely like a voltage source,
producing a voltage waveform approaching a square wave, while the
series-input low-pass filter takes a roughly sine-shaped current. So
this could be considered a form of class-F operation.
At this time I have a test transmitter up and running, using just the
driver FETs (2x RD16HHF1) as an experimental ET final stage. It works
very well, producing roughly 70% efficiency over the complete SSB
envelope, compared to 20% or so when running in linear class-AB. At the
same time the IMD performance is way better than in linear class-AB
mode! Like -45dB for IMD3, and dropping from there.
The RD16HHF1 has comparatively high RDSon. The MRF1K50N is better in
this regard, relative to its voltage and current ratings, so I expect
something closer to 80% efficiency from it - and that's consistent with
information given in its datasheet.
But I foresee one problem, about which I would like to hear any good ideas:
While a series-input low-pass filter has high impedance at all
harmonics, any line length between the MOSFETs and the filter will
introduce impedance transformations, so that the MOSFET drains will see
various different load impedances at the various harmonics of the
various bands. If bad luck hits, on some harmonic of some band the load
impedance might even be a dead short. And that worries me.
My RD16 test amplifier is quite immune to this, because it has a very
small conventional output transformer. So the total wire length there is
small, introducing no problematic impedance transformation at harmonic
frequencies. But the legal-limit LDMOSFET version necessarily will have
much larger transmission-line transformers, and the undesirable
impedance transformation there will be highly significant. Assuming that
the total cable length from the LDMOSFET drains to the low-pass filter
will be 70cm, that's an electric length of roughly 1 meter, and that
will introduce significant impedance transformations for all frequencies
from about 30MHz up. And that means that all harmonics of all bands from
30m up will be affected, while the lower bands will have at least one of
a few of the most powerful harmonics exempted from this problem.
So, does anyone of you know how bad the effect of this will be? How much
efficiency reduction it will cause? How much risk to blow the expensive
LDMOSFET (I bought just one, and don't want to blow it up while
experimenting!)
And does anybody have a good idea how to manage this? Maybe by
empirically tuning each low-pass filter? That would absolutely require
one filter per band, rather than sharing filters between neighboring bands.
Or maybe somebody has a good idea about how to match the LDMOSFET's
drain impedance to 50 ohm while using the least possible wire length? It
must be a broadband solution, of course, because bandswitching directly
at the drains isn't feasible, given their low impedances.
Manfred
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