-----Original Message-----
From: Johan Bodin <[email protected]>
To: [email protected] <[email protected]>
Date: 08 December 2000 17:42
Subject: LF: SV: RE: Re:amplifiers
Andy and All
The parasitic diode is no good in this configuration for catching the
spikes from what I call the transient soak choke, I use separate clamp
diodes up to the rails effectively 'crosing over' the centre tapped
choke - see the circuit in the new LF Experimenters Book.
I see. Yes, the MOSFET parasitic diodes are known to be slow. I will get
a copy of the new LF book and have a look at the circuit.
By my reckoning no extra filtering is then necessary as second harmonic
is practically non existant due to the square wave drive, and third
harmonic is 26dB down to start with due to the tank and then seriously
reduced further by the antenna loading coil.
True, but watch out for HF, and even VHF, radiation from the wire between
the FETs and the series tuned circuit. I mesaured rise times of a few tens
of nanoseconds on my experimental 4 * IRF640 H-bridge and the harmonics
were strong even on my 144 MHz receiver! Well, the PA was breadboarded
and the "hot wire" was half a meter long. A metal case enclosing the PA and
the series circuit will probably help...
I also have it on good authority from a transmitter designer, that the
classic PI filter with input C is VERY bad news for using at the output
of MOSFET PAs.
Class D PAs come in two flavours. Current switching and Voltage switching.
Full and half H-bridges (the Decca TX has this configuration I guess?) are
voltage switching amplifiers where the drain voltage is a square wave and
the fundamental frequency energy is sucked out by a series resonant circuit
to the load. The transistor current is sinusoidal for obvious reasons. One
half
period per transistor. Yes, a C input filter is really bad news in this case
as it
would lead to infinite current spikes if all components were ideal...
Current switching PAs, on the other hand, works the other way around. The
current through the transistors is a square wave (a constant DC current that
is alternating between the devices) while the voltage (across the "off"
transistor)
is sinusoidal. This kind of PA *NEEDS* a C input filter, or a parallel tuned
tank.
The choke feeding the center tap of the output transformer is very important
since it keeps the DC current constant over the RF period (the voltage at
the
center tap is NOT a steady DC!) This circuit is probably more likely to blow
FETs than the voltage switching circuit. I can imagine two reasons for this:
1 - the DC feeding choke (and the leak inductance in the transformer) will
kick
up the voltage during the drive gap, 2 - a load mismatch may kill the
transistors
with excessive voltage (depending on LPF design). Some kind of "spike
eaters" such as Zobel networks (R+C) are necessary to protect the
transistors.
The G3YXM design is a well known example of a current switching PA.
73
Johan SM6LKM
While my observations would agree with the above facts. Care without a lot
of inbuilt protection is necessary. I use the YXM approach and todate have
never blown any fets at power out levels of 800w. My equipment is all home
built and arranged to match 50 ohms throughout the system. I tune up using
the SWR method ie min ref in conjunction with max forward. I DO NOT tune for
max SMOKE/RF OUT. This is FATAL to FETS in the PA. When tuning up with an
ammeter in cct but using the SWR method swinging the resonsant point either
side of resonance will indicate an increase in RF out on the ammeter also if
you are monitoring the DC amps applied to the FETS it will increase to
dangerous levels ie BLOW UP. I know some by their own admittance only use
the tune for max smoke method and blow FETS GALORE. Only when they have been
converted to the swr method have they ceased to blow up the PA'S.
This info should be OLD HAT but it is surprising how many LFers and others
that should know better have been caught out. Tube amps are more rugged and
will normally stand the strain, at least you get some warning and adjust
accordingly.
Fets just silently go.
ALSO
What person with any sense would operate into an erratic load where the swr
is fluctuating wildly ie shabby antenna blowing about in the wind. If you
blow fets under these circumstances you do not deserve sympathy.
Use a stable antenna or go qrt under adverse wx conditions.
G3KEV
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