AM Basics & Station Setup
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Output Circuit Values & MOSFET ratings
Gate Drive & Drivers
High Power & Harmonic Reduction
Testing & Tuning Procedures
Modulators & Power Supplies
Simple 400 Watt
RF Amp for
VFO for 160 & 80 meters
Pulse Width Modulator and power supply
24 MOSFET RF Amplifier - Step by Step
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Reducing Harmonics and Getting Higher Power
Modular class E RF amplifier using two 5 MOSFET modules, designed for
low harmonic output. Built by Bob, K1KBW.
Modularization involves the use of two or more basic class E RF amplifiers
connected to a single output network. Generally, each module will have its own
input transformer(s) [or driver ICs if this is used], and output transformer.
The voltage outputs of the individual
modules add together, and the impedance is similarly multiplied by the
number of modules connected in series. If many modules are thus connected,
care must be taken to ensure that the wiring used in the output transformers
has sufficient insulation to withstand the higher voltage.
The power output obtained from an RF amplifier of modular design is equal to the
sum of the power output of each of the individual modules. It is possible
to combine many modules to form one large RF amplifier.
The output of a standard class E RF amplifier, as delivered to the output network
is a very non-symmetrical, harmonic rich waveform. By using an even number of
modules, it is possible to configure the amplifier to deliver a symmetrical, rounded
waveform to the output network, which contains significantly fewer harmonics.
This is accomplished by connecting the modules out of phase with respect to each other.
The modules are driven out of phase with respect to each other, and the
outputs are also connected out of phase (reverse the secondaries of the RF output
transformerss of every other module). This type of connection is sometimes
called single ended push pull.
When the module or modules in phase "one" are turing on (gates of the MOSFETs
driven positively), the modules in phase "two" are turning off (gates
driven negatively). Each module will produce a standard class E output waveform,
and the output waveforms will be out of phase with each other.
When one module is turing on,
and producing the flat or square portion of the
class E waveform, the other module will be turning off, producing the high output
pulse of the class E waveform. Since the secondaries of the output transformers
are connected in series and out of phase with each other (and the inputs are
being driven out of phase with respect to each other), the resulting waveform
is symmetrical. One module (or set of modules)
will provide the positive portion of the cycle, the
other module or modules pding the negative portion of the cycle.
This diagram shows a Balanced and Modular Class E RF Amplifier
Note: The modular circuits shown here do not show exact values for some components.
I do not have actual values, as I have not personally constructed these
exact circuits. You will need to determine the values for the driver
transformer by experimentation. You should have at least 12V positive
peak (24V peak to peak) at the gates of the RF output MOSFETs. The
values shown for the RF output network are approximate, for 75 meter
Getting Higher Power
The easiest way to get higher power with class E amplifiers is to combine
multiple smaller class E amplifiers (modules) together to form one
larger amplifier. An even number of modules should be used, and they
should be configured such that every other module is out of phase with
its neighbor. This will result in low harmonic output, and easier output
The DC drain current of each module should be independently metered. You will
not be able to properly balance the current of each stage without proper metering.
Each module should include TVS devices across the gate bus, drain bus and
modulated DC input.
Balancing multi-module class E amplifiers
The dc current of the modules are balanced with respect to each other by
ensuring each module is operating in the same (or 180 degrees out of) phase.
Small variations in inductance,
lead length, input transformers and gate capacitace can effect the phase
of the input signals, causing an imbalance. If you are using input transformers,
it is usually sufficient to squeeze the turns on the transformer primaries
closer together, or spread them apart to achieve balance.
If you are using ICs and digital drive (no transformers), provisions should be made in the
low level digital circuitry to adjust the balance between out of phase
modules. Small variations in
phase can be introduced by using slightly longer or shorter lengths of
cable connecting the low level circuitry to the driver ICs. The picture
to the right shows a class E RF amplifier consisting of 4 modules of
5 MOSFETs per module. The 4 output transformers can be seen in the background,
under a plexiglass sheet added for mechanical stability.