The power stage and output filter schematic
(Click on picture for full size view)
Getting performance out of the amplifier is all to do with the antenna load. It needs to be accurately matched to 50 Ohm, and we�ll deal with that later.
The Schematic
In the previous article, we took the transmitter right up to the gates of the output FETs. The next drawing
shows the output stage and the output filter. It's a very simple design and is completely reliable when
attention is paid to the construction details.
Description
The power stage consists of two fat FETs operating in class D, with zero bias. These are the devices D1 and D2. The sources are grounded, and the gates fed via 10 Ohm resistors passing through holes in the heat sink. Source connections are made by a copper strip grounding plate, directly to the PCB. The drain leads must be short and made of heavy wire. The devices used are STW34NB20 power FETs, although any similar device capable of 200V and 30 Amps would do. You can see the FETs and the grounding arrangement in the next photo.
Looking back at the schematic, power is fed to the FETs via a fuse and a common-mode choke (L1) wound on two ferrite cores. This choke cancels out the drain current so that the ferrite doesn�t saturate, even at very high current, while still appearing as a high impedance at 474 kHz. The bypass capacitors on the 12V supply are very important, especially the 10 uF polypropylene one, as very high RF currents flow here. The capacitor must have low internal impedance and a high current rating.
The fuse holder must be a robust type designed for high current, or it will overheat. Although a 32 mm glass type is shown here, an automotive blade fuse holder and fuse would be best.
Transformer T1, which is designed to match the very low drain impedance up to 50 Ohm, takes the output from the power stage to the filter. Four tubular cores are used, two each side. The primary consists of a single turn made of copper foil fashioned into two tubes, with the secondary winding wound through the tubes. The ends of the tubes are soldered to small pieces of copper-clad board with holes in them. The FET end of the transformer has its copper board cut to isolate the two tubes, to form the FET connections. The other board supports the fuse holder.
In the next picture you can see the ferrite-cored common mode choke and the small copper-clad boards. The output transformer with four cores is under the choke. This photo is of a different amplifier with the heat sink at right angles. The arrangement in the previous photo is better, as you will have easy access to the FETs.
In this photo, you see the output stage from the side. The common mode choke L1 (two cores) sits above the output transformer T1 (four cores), sandwiched between two small copper-clad boards. Right at the front in this view are the two power supply bypass capacitors, 1000 uF above the circuit board, and the square block 10 uF polypropylene capacitor below. The UHF connector here is the transmitter output socket. The power FETs are hidden on the heat sink to the right.
Note the snubber circuits across the primary of T1 to ground (in the schematic). One of these is visible in the above picture. These snubbers suppress high frequency ringing caused by FET switching that occurs due to the leakage inductance of L1 and T1. The more accurately these inductors are wound, the less energy the snubbers will need to dissipate. The snubber resistors can get quite hot.
A 6-pole 500 kHz Cauer low pass filter, using L2, L3 and L4, follows the output transformer. The capacitor across L3 resonates at the third harmonic, giving the filter excellent suppression on the AM broadcast band. With a push-pull design the second harmonic output is insignificant, and requires no special consideration. It is important to orient L2, L3 and L4 in different planes to avoid mutual coupling. You can see how this is done in the pictures above and below.
L2, L3 and L4 are supported on blocks of wood. It looks horrible, but is robust. If you bolted directly to the plastic formers, the formers would need to be longer, and they would flex more than with the arrangement shown. The next photo shows a different view of the output filter inductors. You can also see both snubbers.
The transformers, chokes and coils are all easy to wind. The cores are available from Jaycar, while the air-wound coils L2, L3 and L4 are wound on small lengths of PVC water pipe. The wire is secured with PVC cement. The use of high voltage polystyrene capacitors here is vital. They see up to 100 V RMS and rather high circulating current. These may be the most difficult of all the components to find. You can see some of these capacitors in the first photo.
The two amplifiers shown in the photos were both built by Graham ZL1QM, who designed the amplifier. One of the transmitters is in my shack, the other with ZL2AFP. Graham has a third one to the same design. All credit and cudos goes to Graham, who conceived the design and layout. He also designed a 10 W version with an identical driver and a pair of IRF510 FETs in the output stage. But life's too short for QRP!
Firing up the amplifier
The next page will cover the exciting business of getting the amplifier running and tuned up. This
involves making your antenna match 50 Ohm, operating first on low power, and how to operate high power without tripping or damaging your power supply. Suitable power supplies will be suggested.
We�ll also talk about suitable antennae for transmitting and receiving, and the modes you might expect to use. With a transverter the choice of modes is very wide, but the non-linear operation of the power amplifier will limit you to CW or single-carrier MFSK modes. No PSK31, SSB or AM. But that won�t hold you back!