From: WarmSpgs@aol.com
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Date: Tue, 16 May 2000 02:30:32 EDT
Subject: LF: Re: LF power amplifiers
To: rsgb_lf_group@blacksheep.org
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I'm not sure that I would agree a single-ended push-pull output is 
significantly easier to bias; linear biasing of "floating" transistors is 
done all the time in audio power amplifers, such as are frequently used by 
hams at LF; but within the stated parameters of a typical Class D design, 
it's not a problem anyway.

As for Peter's question about the bridge configuration, its main advantage is 
the same one exploited in bridged audio amplifiers: four times the power 
output for a given supply voltage and load impedance.  If the power is taken 
out via an RF transformer, EACH end of the primary winding alternates between 
full supply rail and ground.  This is the same as applying twice the voltage 
to a winding with one end grounded, without actually having to stress the 
amplifying devices with twice the voltage.  To achieve the same power in a 
non-bridged arrangement, one must go to a lower load impedance and deal with 
higher currents; or, change the turns ratio and operate the switching devices 
at higher voltage; or some other compromise.

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.

The harmonic filtering requirements for a Class D amplifier are not 
necessarily greater than for a Class C amplifier.  In fact, for single-ended 
C, you have to pay a lot more attention to the second harmonic.  The third, 
fifth, seventh, and ninth harmonics are easier to attenuate than the second.  
That's not to say it's trivial, of course!  Fast switches produce significant 
spurious components up to the edge of UHF, so excellent shielding and 
attention to layout and lead length are important.  The design isn't that 
difficult, but implementation requires attention to detail.

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. 

73,
John KD4IDY