AM Basics & Station Setup
Theory of Operation
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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Gate Drive and Drivers
Selecting a driver for the gates of the final RF amplifier is probably
the most important design aspect of a class E transmitter. Much work has
been done in this area. More than any other part of the transmitter, the
driver will determine the stability and to some extent, the efficiency
of the RF amplifier. Some types of drivers include:
- Direct, sine wave drive of the gates from an external transmitter, transformer coupled
- Sine wave drive using a smaller, internal class E stage, transformer coupled
- Descrete component square wave (digital, non-resonant) driver connected directly to the gates or through a transformer
- Driver IC(s) connected to, and driving the gate(s)
What the Driver Does
The main function of the driver is to supply the RF gate voltage to the
final RF MOSFETs. A second, very important function of the driver is to
hold the gates in either the on state, or off state, and not allow the gates
to "float" under any conditions. If the gates float, parasitics are likely
The RF gate voltage should be around 24 to 30 volts Peak to Peak
(assuming sine wave drive), or alternatively, about 12 volts Positive
This drive level must be maintained during all phases of
modulation. A poor driver will result in poor performance. Remember,
all the gate drive power is converted to heat within the MOSFET. This
factor must be considered with figuring device dissipation, and heat
Digital Gate Drive
Digital gate drive is the best way to drive the gates of MOSEFTs used in
class E amplifiers. There are several advantages to digital gate drive:
The waveform above shows the output of an IXDD414 driver driving 2 FQA11N90
gates. This is a good gate waveform, with no ringing or other anomalies present.
- Broadband - no tuning is required, even when changing bands.
- More Efficient - only positive voltage is supplied to the gates, so less heat is generated.
- The gates are held is the on state or the off state. This significantly reduces the chances of parasitic oscillations.
- Smaller Size.
- Easier construction than analog (transformer coupled) drive systems.
- Phase shift within the driver circuitry can be eliminated, or alternatively, carefully controlled and adjusted.
A driver which delivers only a positive peak
to the MOSFET gates is more efficient than a sine wave driver. This is
because the only useful part of the driving cycle is the positive peak.
The negative peak does not contribute in any way to driving the MOSFET,
however the energy contained in the negative cycle is still converted
to heat within the MOSFET gates.
The gates of the MOSFETs are big capacitors. Keep the leads from the
drive ICs to the gates SHORT (within reason). The gates represent a low impedance at RF. They
can be driven at RF, but you must take care to keep the leads as short
as possible. When paralleling MOSFETs be sure to keep the gate leads
REALLY SHORT, and keep the MOSFETs fairly close together to ensure that
all the MOSFETs are being driven with the same gate voltage, and all
are in the same phase.
Analog (sine wave) Gate Drive
Although digital gate drive is generally simpler to implement, and will usually
yield better results, there are still many applications where analog (sine wave)
gate drive is desired or mandated.
With analog drive, a driver transformer or transformers are usually used to
couple the driver to the gates. A step down ratio is generally employed
to help match the driver to the very low gate impedance. The MOSFET gates
are often connected in parallal, and a wide gate "bus", constructed from
copper or brass strap is used.
The driver itself
can be an external transmitter, or can be "built into" the overall system in
Since the gates of MOSFETs represent SO much capacitance, it is
important to remember that even a relatively short piece of wire or
copper strap will represent sufficient inductance, and can cause an RF phase
shift to occur along the copper strap. If you have a number of MOSFET
gates in parallel along a piece of copper strapping, you need to ensure
that the same amount of drive, in the same phase, is delivered to each
device. The gate bus should be quite wide (up to 1 inch) to reduce stray
Drive Power Requirements
On 75 meters using analog (sine wave) drive, a 10 MOSFET transmitter will require approximately
20-30 watts of power delivered to the gates. Figure about twice this
amount on 40 meters. The important thing to ensure is that you have at
least +12V (24v peak to peak) at the gates of the MOSFETs.
With digital gate drive, figure approximately .5A to .8A at 12VDC driver power
per MOSFET on the 75 meter band.
Maintaining gate drive under modulation: Important!
Depending on the particular circuit layout, the RF gate voltage may
fall as the drain voltage or current is increased, due to coupling
between the output circuitry (including ground loop currents) and the
gate circuit and/or driver. If the RF gate voltage falls too much under
modulation, the device will not be driven to saturation, causing
inefficiency, nonlinearity, destructive parasitic oscillations and
other serious problems. This is usually more apparent with analog
gate drive than with digital gate drive.
There are several high current paths in class E RF amplifiers. One
high current loop exists between the RF bypass capacitor at the DC end
of the primary of the output coupling transformer and the source bus.
Another exists between the shunt capacitor(s) and the source bus. Other
high current RF paths will occur within the output network.
You can generally avoid interaction between RF loops which exist in
the class E amplifier, and you gate circuit, by not using the ground
plane as a the "other conductor" for interconnecting the gate and
Things to consider:
- (Analog Drive) - Solder the "ground" side of gate driver transformer secondaries to the source bus immediately where the sources connect. Don't leave a lot of "space" between the secondary and the sources themselves.
- (Analog Drive) Use twisted, balanced lines or shielded cable between the primaries of the gate driver transformers and the driver, and let the primaries float at the transformer end.
- Keep the RF output circuitry far from any tuned driver circuits.
- (Digital Drive) - When using driver ICs, terminate the cable(s) carrying input signal to the driver IC(s) with a resistor, and keep the leads short.
- (Digital Drive) - Use a separate, short (a few inches) shielded cable for each driver IC, terminated at the IC, and brought back to a common feed point.
Types of Drivers
Digital Gate Drive using a Single or Multiple Driver ICs
One very practical way to provide gate drive to the final RF amplifier is
to use one of the several RF driver ICs available for the purpose. Some of
these driver ICs will operate on 30 mHz or higher. If you chose the correct
driver IC, it is possible to provide almost square wave drive to the gates of the RF amplifier MOSFETs,
which can result in more efficiency and will significantly reduce the possibly
of parasitic oscillations in the RF amplifier. Since the driver is not tuned,
it is possible to operate the driver on multiple bands, making multi band
operation much simpler. The driver ICs generally take a standard 5V TTL input
As of this writing, IXYS / DEI appear to provide the most robust driver ICs with
respect to RF service. The IXDD414 will drive a single FQA11N90 MOSFET quite well
at 7 mHz and below. These devices cost approximately $3.00 each. One driver IC
must be provided for each output MOSFET. The DEI DEIC420 will drive up to
5 FQA11N90 MOSFETs on 80 and 160 meters. The DEIC420 currently costs around
Using a DEIC420 (DEI / IXYS)
The DEIC420 is an rf packaged, high current, high frequency driver IC. It is
designed to work up to 45mHz into a few thousand PF or so, but will work
at lower frequencies into 15000pF or more. The input capacitance 5 FQA11N90 MOSFETs in parallel
will add up to about 15000pF. The DEIC420 will drive the 5 MOSFETs very well
up to 4mHz, making this a good driver for 80 and 160 meters.
a single driver IC is used per bank of MOSFETs, construction is relatively simple
and straight-forward. The IC requires 12 volts at over an Ampere. It is
recommended that you use a switching regulator such as the LM2576 rather than
a linear regulator. Provide a TTL signal to the IC's input at the operating frequency, and be
sure to properly terminate the coax cable carrying the input signal with a
100 ohm resistor to ground, and a 100 ohm resistor to a locally bypassed +5 volt source.
The IC must be well bypassed, as it is a high current, low impedance device. Use
more than one low impedance bypass capacitor of at least .1uF to .5uF per IC.
The main disadvantage of the DEIC420 is the cost - around
$31.00 for a single part! Perhaps IXYS / DEI will consider a price reduction
for Amateurs using their products!!
Using Multiple IXDD414 Driver ICs
The IXDD414 is a small, 5 pin TO220 packaged device capable of operating at
least up to the 40 meter band into a single FQA11N90 MOSFET. One practical
configuration involves driving 5 of the IXDD414 devices in parallel, and
connecting the output of each device to one gate of an FQA11N90 MOSFET, with
the MOSFET drains and sources connected in parallal. So, the inputs to the
drivers are in parallel, and the drains of the output MOSFETs are in parallel,
but the gates are not directly connected in parallel, but are driven identically
by the driver ICs.
When using multiple driver ICs in a single class E stage, care must be taken
to ensre each driver IC receives the same input signal, and in exactly the same
phase. The most reliable method of accomplishing drive to the driver ICs is
to run a separate, short shielded cable to each driver IC, brought back to a
centeral point, and then fed using one piece of shielded cable.
Another method that can also work is to create a gate driver input "bus" from copper strap.
The physical construction should be as symmetrical as possible. Take care to
avoid any coupling between the driver IC input bus and the output, or parasitic
oscillations may result.
Analog Drive Systems
Driving the gates with an external transmitter
If you already own an RF source capable of supplying 30 or so watts of power,
you can use this transmitter to drive a 10 MOSFET transmitter on 75 and
160 meters. If the driver will put out 50 or 60 watts of power, you can use
it on 40 meters as well.
Generally, since you are looking at a very low impedance when
driving the gates of MOSFETs, it is desirable to use a transformer to
step up the gate impedance to make it easier to drive with conventional
sources. A ratio of between 4:1 and 6:1 works very well for driving the
gates of MOSFETs. A single turn secondary is all that is necessary. Use
type 43 material for 160, 80 and 40 meters. The FB-43-1020 is a very
good core for the purpose. They cost approximately $2.50 each and are
available from CWS - BYTEMARK or Amidon. A single core is generally all
that is necessary for the driver transformer. There are many examples
and pictures of driver transformers presented in the RF amplifier
section of this writing.
You will need to provide some kind of matching network between the driver
and the primaries of the gate transformers. An L network works very well
in this application. Most modern transmitters will not work well, or at all,
into a mismatched load. Proper matching between an external driver and the
load is essential.
Using a Dedicated Internal Driver
The procedure for using a dedicated driver is similar to using an external
transmitter for driving the gates. The driver itself will require a driver
of some type (perhaps an external transmitter or driver IC). In essence,
the driver is really just another class E, D, or C amplifier used to driver
a larger amplifier stage.
If the driver uses a resonant output (such as with class E or class C drivers),
the driver will need to be tuned correctly for the operating frequency.
When using an intermediate driver that uses a resonant output, be sure the
output of the driver is stable over the entire band you wish to use. In general,
class E amplifiers deliver more output as the frequency is reduced. Make sure
you are not overdriving the gates in the low part of the band, and/or underdriving
the gates on the high end of the band.