Dick PA0SE and others,
An interesting contribution. My pushpull
bipolar QRO PA can run to near squarewave performance, and I have successfully
used an inductor input low pass filter for many years. The low pass filter
uses three large potcore inductors and HV polystyrene capacitors. Texts on
Class D and E do mention resonant circuits, and most of the commercial
applications are in narrow bands, so tuning is a reasonable way to implement
Class D/E for those users. At LF I believe a hard driven final can
simply be followed by a low pass filter and still deliver very high efficiency
(certainly a lot better than Class C). In New Zealand our band is about to
be expanded to become 130 - 190 kHz, which is a relatively wide band, and not
worth having tuning that needs revisiting for significant frequency
change. A broadband PA with a selection in low pass filtering is my
preference. For use only in the 136 kHz band, only one low pass filter is
My preference for an inductor input filter came
from reading (I hope my recollection is right) a Motorola article on stability
of HF solid state power amplifiers, and the preference was for an inductor input
filter. I merely scaled the filter values to LF and found they worked
well. Possibly the instability bogey is not applicable to LF but I was
aware that inrush current would be different if a capacitor input filter was
used. However, there are polystyrene capacitors from collector to ground
so on the PA side of the output transformer there are "shunt capacitors" and
these certainly reduce the "high frequency nurgles" as seen on a scope
I have not tried "snubber" series RC networks but I
have seen commerial high power switchmode inverters that use that, and the
resistors are surface mount with good power rating. Some of that 16%
"harmonic power" gets absorbed in the interests of controlling spike
Power devices (bipolar of MOSFET) can have slower
turn-off than turn-on, and I suspect this has a large impact on transients in a
pushpull power amplifier.
Regarding suppression of harmonics, and potential
radiation, the efficiency of amateur (electrically small)
antennas rises significantly (more than
linear) with increasing frequency. So merely a loading coil or "net
antenna Q" is only part of the consideration of radiation of harmonics.
The insertion loss of a low pass filter can be kept fairly low and I think it is
best to include it in a transmitter design, especially a hard driven
73, Bob ZL2CA
----- Original Message -----
Sent: Tuesday, June 08, 2004 8:06
Subject: LF: Effect of LP-filter om
To All from PA0SE
subject may have been discussed on the reflector before but I can't remember
Class D and E final amplifiers have high efficiency because they
produce square waves. When the voltage between source and drain of
the FETs is high, current is zero; when current flows voltage is almost
But what happens if the transmitter is followed by a low pass filter
with a shunt capacitor at the input, as is often the case?
Every cycle the
charge in the capacitor must be reversed and with a current of finite value
that takes some time. During that time current flows in the conducting
transistor whilst voltage over it is not small. So one can expect dissipation
in the transistor to increase and efficiency to decrease.
Looking at it in the frequency domain one could say that the shunt
capacitor causes the square wave to start becoming a sine wave.
current into or out of the capacitor is so high that charge reversal takes
negligible time, so the square wave is preserved, then output resistance of
the transmitter is obviously so small it almost puts a short circuit over the
capacitor. In that case the capacitor does nearly nothing and could just as
well be deleted...
The problem does not arise when the LP-filter
starts with a series inductor. The sudden cut-off of current would result
in a very high kick-back voltage at the drain of a single-ended
amplifier, almost certain killing the transistor.
But in a push pull
amplifier this is prevented by the other transistor that starts to conduct at
the same moment.
Provided of course coupling between the two halves of the
primary winding is very tight.
So it seems to me that class D and E
amplifiers should preferably be followed by a low pass filter with a series
inductor at the transmitter side.
Please correct me if my
conclusion is not valid.
An interesting question is whether a low pass
filter is really necessary when the transmitter feeds the aerial via a series
Harry Grimbergen, PA0LQ, has given this some thought and he
says the following (my translation).
Not unusual is an aerial
capacitance of about 300 pF and a total loss resistance in coil, aerial and
earth of some 40 ohms At 136 kHz this results in a Q of about 100 for
the aerial system. At the third harmonic this produces an extra
attenuation of about 280 times, or 49 dB. The third harmonic of a square
wave is 9.5 dB weaker than the basic frequency. On the other hand radiation
resistance increases with frequency squared; 9 dB for d3.
harmonics will be suppressed almost 50 dB.
About 16% of the power in a
square wave is in the harmonics and this power is reflected back into the
But it is not converted into heat. The DC input is
decreased by this 16% as compared to the situation where the amplifier is
loaded by a resistive load of 40 ohms for the above case. I have been
able to show this also using simulation by MICROCAP.
problem in The Netherlands would be that the radio inspector does not
measure harmonics as field strength but as power in the output of the
transmitter (or LP-filter, when present). So selectivity of the aerial system
does not help.
Any comments will be very welcome.