A collection of modules for building medium to high power Amplitude Modulation (AM) transmitters. Each module includes a schematic PDF, essential construction details, and many include links to Gerbers for the PCB.

Module AM#1: Triple DC buck regulator

A PCB design for three variable voltage 0..35 volt, 3 amp buck regulators, using the LM2576 switching regulator, and designed for use in a homebrew AM transmitter, but generally useful. Unwanted sections of the board may be hacksawed away if only one or two regulators are needed in a project.

Gerbers, BoM and other supporting files are available in this Github repository.

Module AM#2: 100VDC (500 watt) buck regulator

A schematic and PCB design for a DC power buck regulator capable of delivering 0..100 volts DC from an input of up to 120 volts DC at 3 to 5 amps continuous, and at peaks of 2 to 3 times. Designed as the main power supply regulator for a 250 watt AM transmitter.

Gerbers, BoM and other supporting files are available in this Github repository.

Module AM#3: Digital SWR and power meter

A schematic and PCB design for a simple digital RF power and SWR meter. The following video demonstrates the meter:

Gerbers, BoM and other supporting files: email me.

Module AM#4: Balanced microphone amplifier with phantom power and equalisation

A schematic and PCB design for a balanced microphone amplifier with phantom power and equalisation, and using all discrete transistors. The module runs on a single 12 to 18 volt DC supply.

Gerbers, BoM and other supporting files: email me.

Prototype balanced microphone preamp with 12v phantom power and hi-mid-low equalisation.

Module AM#5: H-Bridge Class D RF Power Amplifier

This module is an H-Bridge class D switch, not a power amplifier in the pure sense as it is non-linear, rather a switching module capable of delivering over 100 watts of power into a 50 ohm load, or other loads with a different output transformer turns/impedance ratios.

The FETs are Infineon devices (IRFP4019) aimed at classs D audio amplifiers, and cost a few dollars each (if a supply can be found, as of Apr 2022 due to the global semiconductor shortage). Better performance is delivered by another Infineon FET, the IPP530N15 (out of stock globally at time of writing ).

The module includes a gate driver (IXDD614, still available), which can be driven with a 5V TTL square wave from a crystal oscilator, synthesiser or PLL, followed by a 74HC-series TTL buffer. This selection of driver and FETs has worked with an efficiency of around 90% from LF to 7MHz.

A subsequent set of photos will illustrate the angle aluminium heatsink piece using 50x25x3mm stock angle. Two of these PCBs fit side by side under a 150x60x25mm finned aluminium heatsink, commonly available on aliExpress, eBay and other sources. These H-Bridge modules may be combined for higher power levels, either by using an RF power combiner (W8JI has an excellent one), or by arranging the secondaries in series whilst adjusting their impedance so as to sum to 50 ohms.

Gerbers, BoM and other supporting files: email me.

A pair of H-Bridge modules in a test rig.

Module AM#6: Universal VFO Drive

This module takes as input a 3 to 5 volt square wave clock (such as a VFO generated by a crystal oscillator and buffer, si5351 multisynth PLL or similar source), and delivers one, two or four equivalent drive signals (I/Q or I/Q/I-bar/Q-bar). Outputs may be configured to be all in phase, or in some combination of 90 degree shifted phases. The input frequency may be at the desired frequency or at twice the desired frequency. An TTL-level enable line is provided for (on/off) drive control.

A schmitt trigger may be added to square up an imperfect drive clock. This board is intended to be used in a multiphase AM transmitter, to drive one, two or four H-Bridge modules, but is universally useful for any projects that require VFO/clock conditioning, with or without frequency division, or up to four phase-differentiated outputs.

Gerbers, BoM and other supporting files: email me.

Prototype board used as a driver for 2 or 4 modules.

Module AM#7: PDM Driver and LPF

This module takes an HT supply in the range 0..100 volts DC, and a modulated pulse stream at the chosen Pulse Duration Modulation frequency, and performs power switching into a low pass filter. The result is a modulated DC voltage, suitable for powering an H-Bridge module to generate high quality Amplitude Modulation. This module is sized to power and fully modulate up to two of the H-Bridge modules (module #5).

Switching is done with a pair of Infineon IRFP260 power FETs, driven by a high and low side IR2110 gate driver. A modulated PDM (PWM) signal at 5V is required at the input. A TTL-level enable control turns the switching stage on or off, for transmit/receive switching, or to drop the (modulated) DC output to subsequent H-Bridge(s). An Over-sample line provides a degree of over-sample error correction back to the multi-phase modulator (to be described).

The Low Pass Filter is designed for a PWM clock (sampling) frequency of 100 to 125kHz). It has a -3dB cutoff at around 28kHz with the components as specified. Inductors are wound with 1mm ECW on RM10 cores. Capacitors must be metallised polyester (MKT) and rated at 200V for a DC supply of 100VDC. Electrolytic capacitors are Low ESR.

The PCB is identical in side, mounting holes and heatsinking arrangement to the H-Bridge module. One of these modules will power one or two H-Bridge modules, for a conservative 150 to 200 watts of fully modulated AM carrier power. With two H-Bridges running 200 watts carrier power, passive cooling using a 150x60x25 heatsink on each of the H-Bridges and the PDM Driver module without a fan will suffice. Two of these modules will power four H-Bridge modules, for up to 400 watts of fully modulated AM carrier power.

If using two modulators, the PDM drive may be supplied to each modulator in phase, or at 180 degrees for a multiphase configuration (using module #8). It is possible to use four of these modulators and four H-Bridges, and drive each with 90 degree phase shifted PDM drive, for a true quad-phase modulation configuration. This is probably unnecessary but would make an interesting test!

Gerbers, BoM and other supporting files: email me.

Prototype PDM Driver and LPF board in a test rig.

Module AM#8: Multiphase modulator

This experimental module contains a crystal oscillator and divider, followed by four Miller Integrators and Linear Comparators to deliver one, two or four audio modulated Pulse Duration Modulated clocks at the selected PDM base frequency (typically 250 or 125kHz). To be described.

Prototype board with four PDM 90 degree phase shifted outputs.

Module AM#9: RF Power Combiner

To be described.

Module AM#10: Triple Low Pass Filters

This module is a set of identical low pass filters with relay switching for an AM transmitter or high power linear amplifier. The board is designed for components in the 300 watt continuous carrier range, and is physically arranged so that it may be cut down for a single or pair of LPFs.

To be described.

Prototype LPF board for an AM transmitter or linear, with filters for 160 and 80m populated.

A complete single phase 150W AM transmitter

Pictured is a complete AM transmitter for 160 and 80m using the modules described above. It was used as a test rig for the modules, and after the RF Power and LPF boards were added, was used on-air as shown with good results on both bands.

Test rig for the modules.

HT for the rig was supplied by two 30V 5A bench power supplies in series ( not shown). A separate 12V supply powered all of the 12V circuitry. These supplies provided HT current limiting. No SWR protection was used. A digital scope was used for signal and modulation monitoring and RF output power measurements.

To complete this transmitter would require an HT DC supply capable of 120V DC at 5A continuous, followed by the 100V DC Regulator module above. This module incorporates preset DC current limiting. The SWR meter shown above would monitor SWR and antenna match, as well as a high SWR lock-out line back to the Regulator.

The next experiment to be tried on this test rig is to use two PDM phases to drive two modulators and two RF Power modules, either in series, or, into an RF power combiner. This should deliver 250 watts of AM at close to 90% efficiency without any significant heatsinking and without fans.

5 thoughts on “AM-Transmitter-Modules

  1. Paolo says:

    congratulations for your very interesting web site!
    Of course I need ( as always…) some more info, especially about AM#5 H bridge class E rf power ampli:
    As connected, both high side and both low side of H bridges will go on at the same time , IMHO two trasformer must be inverted ( primary or secondary) on one side of H bridge: I’m I wrong?
    Can I use only 2 transformers on gates with 3 winding each?
    Some more infos on out transformer: in your full schematic you write: 8T bifilar 1T primary 2T secondary: can you explain?
    Sorry for my terrible english!
    Thank you
    Paolo from Italy

    Liked by 1 person

    • Paul Taylor says:

      Hi Paolo,
      thanks for commenting. You are very observant, and you are correct, when I built up this board, the drive was wrong across the four gates, and it wasn’t until I reversed two of the gate transformer primaries that it worked. If you look carefully at the coloured insulated primaries on the left-most two transformers in the photos or video, you will see that they cross over each other.

      I always fix the schematic but I must have omitted to re-publish the fixed PDF on my blog.
      The H-Bridge circuit is simple and you can easily experiment with the key parts. I’ve tried different FETs, changing the output transformer turns ratio, and I’ve omitted the modulated HT decoupling capacitor. I’ve not experimented much with the gate drive or gate transformers, other than trying FT50-43 which worked, but got too hot. I cannot think of a reason why you couldn’t use two gate transformers with two secondaries, there tend to get hot, so obviously pay careful attention to getting the phasing correct, and monitor toroid temperature.

      The IXDD614’s output can overdrive the gate transformers at lower frequencies. You can address this by dropping the IXDD614 Vcc from 12v to 10v or even 8v, while monitoring the H-bridge output waveform. Keep reducing Vcc until the output wave just starts to deteriorate (lose its square edges) . The IXDD614 will run a lot cooler if you do this, another bonus.

      If you do experiment with this module, let me know what you find. It is an excellent design, >90% efficient, can work 630 to 40m, up to 250 watts carrier power, reliable as long as the gate drive is set up correctly.

      73 Paul VK3HN.


    • Paul Taylor says:

      Paolo, I forgot to clarify your question on the output transformer. Another schematic error 😧 It should be 8Turns Trifilar. Use one turn as the primary, put the other 2 turns in series for the secondary. I have used both a single T140-61, and a pair of stacked T114-61 toroids. They seem to be about equivalent.


      • paolo says:

        Hi VK3NH,
        thank you for support and good ideas!!!!!
        If primary is 8 turns and secondary is 16 turns ( 1:2) so impedance is
        1:4,  and primary is 12.5 ohm.
        If I want to have more power and I use two circuits in series, so
        secondary must ( I suppose….)
        be only 25 ohm each, right? so 1:2 impedance, that is 1: 1.4 turns say
        about 8: 11 turns. I never did this before,
        but I want to try. I’m not a broadcaster, but I will only heat up my 50
        ohm dummy load and listen
        modulation on an old Marantz radio.
        Lot of good ideas on your web site, I’m an old 66 y.o. technician, and I
        hope to have time to test them all.
        Thank you for your web site and support.
        Sorry for my very primitive english…..
        Paolo from Italy


      • Paul Taylor says:

        Hi Paolo,
        Don’t apologise for your English, I think you are far smarter than me, I speak no Italian 😉

        Regarding, putting two modules together to get twice the power — I believe your suggestion should work. Build two boards, use 8T primary and 11 turn secondary, and put the secondaries in series. Drive both modules with the same clock or 5Vpp signal. Use your Modulator to modulate the HT to both modules. I have thought about doing this but have never tried it.

        One of the K7DYY AM transmitters does exactly this. I went looking for his schematic but I cannot find it, he has probably taken it down.

        When combining the power from two of these modules, I have used the W8JI RF power combiner with success. See schematics on my 200w AM Transmitter post. W8JI has an excellent page on this, read it carefully for the theory of operation.

        It presents a 50 ohm load on both input ports, and delivers the combined power into a 25 ohm load, which you can step up to 50 ohms with a simple transformer. The cool thing is that if one module fails the transmitter keeps working at half power. It is also simple to build and requires no adjustments. You could try substitute toroids, but you will need two high power 50r stud resistors, available on eBay fairly cheaply. Most of the work in this module is mechanical mounting.

        73 Paolo and let me know if you get this working. Paul VK3HN.


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