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From: "Talbot Andrew" <ACTALBOT@dera.gov.uk>
To: rsgb_lf_group@blacksheep.org
Subject: LF: Re: LF power amplifiers
Date: Tue, 16 May 2000 09:44:20 +0100
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> Referring to John KD4IDY's note -
> 
> I cannot help but feel a bit uneasy about the use of class D
> amplifiers followed by conventional shunt C series L pi-section
> filters - although the designs using them do seem to work OK.  The
> shunt C is surely going to increase the circulating current at the
> harmonic frequencies, stressing the active devices more an lowering
> efficiency.  I would have thought that the first component of the
> output filter should be an inductor - either giving a Tee section
> filter, or Pi section with an even number of elements.  Dave / Dave
> can you comment further ?.   
> 
> This approach is preferred by G8GSQ who has built high power HF Mosfet
> PAs and finds the initial L in the output filter improves device
> stability considerably.  QEX ran an article on HF PA filters a few
> months ago, and these ended up as quite complex diplexing units in
> order to present the devices with respectable out of band impedances.
> A tank circuit actually sounds quite attractive on first thoughts just
> as a filtering element, but unlike a valve, we are dealing with much
> lower impedances.  Back in the valve class C PA days, (I'm just old
> enough to remember these from several Practical Wireless articles)
> seem to recall a loaded Q of 12 being taken as optimum.  To get this
> sort of Q at a 50V rail 10A current needs a seriously small value of
> inductor in a conventional tank.
> 
> A back of envelope calculation :
> 
> If  Rload = 50V / 10A = 2 ohms and   Ql = 12  then     Xl = 2 / 12 =
> 0.17 ohms  = 194nH at 137kHz.  Which is one turn (or less) on a decent
> sized iron dust former or a few turns of VERY thick wire in air.
> Furthermore, with I = 10A and Q = 12 then circulating currents
> approach 120 Amps.  Enough said.    In practice, tapping into the tank
> circuit would be used and the easist way to achieve this is to use two
> capacitors rather than try to tap into turns with high current wire,
> so now a choke is needed to feed in DC.  An alternative to tapping is
> to use a transformer, which also has the advantage of isolating the
> antenna circuit from the PA at DC.   We now have a transformer PA
> followed by a resonant network just providing filtering - Beginning to
> look familiar ?
> 
> In the early days of 73kHz I tried this approach, but the high Q
> output network was too prone to destroying devices when mistuned, and
> particularly when the antenna went off resonance.  The ultimate design
> was a half bridge circuit running off 340 Volt rectified mains, output
> transformer and C/L tank using an air wound inductor rather than
> conventional Pi filter.  For a time it delivered 700 Watts into a
> dummy load of multiple 150W light bulbs quite happily - until I keyed
> the circuit a few times.  Remember what the resistance of light bulbs
> are when cold and the current surges when switched on.  There is a lot
> of energy in 1000uF of capacitors charged to 340V.  Never did like CW
> !   After blowing four IRF450 fets spectacularly plus two bridge
> driver chips I decided this was rather dangerous and kept with
> conventional class B designs from then on.   But now that really high
> power is wanted, it may be time to look at using rectified mains
> again.  The output transformer provides one side of a safety barrier
> and input isolation can come from a transformer or opto isolators.
> An input transformer is preferable as it protects preceeding devices
> from large DC voltages/currents generated as a result of FETs
> exploding.  Switch mode PSUs have been doing this for a long time with
> a good safety record and there is a large base of SMPSU components
> both surplus and in the catalogues.  Everyone uses SMPSUs these days.
> 
> I don't have a death wish, so will leave such a PA  to others to
> develop.
> 
> On the last point of John's note...
> 
> At the moment I'm developing a Pulse Width Modulator for supplying
> Class D PAs.  This will accept a binary word (which can of course be
> the output of a A/D converter) and control the supply voltage to the
> PA.  The aim is for a supply which will operate with 50V input and
> supply 0 - 50V out at currents up to 10A.   Software authors could
> hopefully be persuaded to make phase and amplitude available
> separately, digitally, or these could be regenerated as per John's
> suggestion.  The PWM scheme is open loop so can operate very fast -
> I'm using a switching frequency of 50kHz but this is not critical.
> Modulation up to a few kHz should be feasible.  Works OK so far at up
> to 3.5 Amps output, but I haven't tested at full current yet not
> having had any suitable switching devices in the 'junk box', but I
> hope to in a few days time, so watch this space.........
> 
> I already have a PIC/EEPROM based  PSK31 beacon which supplies
> separate amplitude and phase, and have tested this successfully on
> 1.8MHz at 8 watts, but with a 12V supply rail just being very
> inefficiently modulated with an emitter follower - thus negating all
> the efficiency advantages of a class C/D PA !  It proved the concept
> though.
> 
> Andy  G4JNT
> 
> From  John KD4IDY
> 
> 
> >The output of any dual-ended or bridged switching amplifier does not
> require 
> >the flywheel effect of a traditional tank circuit.  The output of the
> 
> amplifier is simply a square wave, from which everything but the
> fundamental 
> can and should be stripped (with impedance transformation, where
> necessary).  
> Unlike single-ended switches, which require resonant circuits with a
> certain 
> minimum loaded Q to opreate at all, dual-ended switching amplifiers
> can be 
> built to work over roughly an octave without retuning.
> Side note: If a "linear" amplifier is desired with extremely high
> overall 
> efficiency, one solution at medium power levels is to use 
> envelope-elimination-and-recovery.  (That was the late Helge
> Granville's name 
> for it at Motorola, though I suspect there are other terms in use as
> well.)  
> The signal to be amplified is passed to both an envelope detector and
> a hard 
> limiter.  The hard limiter output becomes the RF drive for the
> switching-mode 
> final amplifier, be it Cass D or E or something else.  The
> baseband/envelope 
> voltage is amplified linearly, although this can be done at high
> efficiency 
> too, in an amplifier switched with pulse duration modulation or other 
> techniques.  The amplified envelope becomes the source voltage for the
> final 
> RF amplifier.  Assuming there are no significant time errors between
> the RF 
> and envelope paths, the output can be a highly accurate representation
> of the 
> input, as the RF path preserves the phase information impressed on the
> 
> carrier and the envelope contains the amplitude information.  All
> modulation 
> schemes involve varying amplitude and/or phase, so the technique works
> for 
> CW, SSB, PSK, AM, QAM, etc.  There are some practical complexities,
> but I 
> know amateurs who have used it successfully at levels of several
> hundred 
> watts. 
> 
> 
> 
> 


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