Class H, High Efficiency Analog Modulator
Combination class H modulator - power supply by Bob, K1KBW.
This implementation uses 10 IRFP260N MOSFETs, 5 in the each leg of the modulator.
The schematic shows 12 Devices (6 MOSFETs and 6 Bipolar Transistors), 6 in each leg. Up to 6 Devices may be used in each leg of the modulator.
This is a class H, analog series modulator. This
design is much more efficient than a standard class A series
modulator, which would typically be around 30 or 35 percent efficient.
The modulator has excellent audio performance, and is stable
and easy to adjust. A system based loosely on this technology was used
in the Harris MW1 Solid State 1KW Broadcast Transmitter. This
technology is applicable to vacuum tube designs as well as other solid
The idea behind class H, and a related class, class G, is to run the
series modulator devices in the audio output at or near saturation.
The voltage supplied to the near saturation devices is increased when greater
output voltage is required. Otherwise, the when a lower output voltage is
needed, the saturated device behaves like any other series modulator.
In class G, the
supply voltage supplied to the almost saturated output amplifier is stepped,
and the number of power supply steps depends
on the particular design. In class H, the supply voltage is adjusted
linearly, rather than in a step function.
To The Right: Class H modulator-power supply built and designed by Steve, WA1QIX.
The boards simplify the construction of the modulator.
A complete board set is now available for the class H modulator and the
Efficiency Meter / Overload Shutdown. Contact me if you're interested.
More detailed Pictures of this class H modulator - power supply combination
Note: there have been several revisions to the class H modulator
since its first version. The current revision corrects several minor
problems, and implements an overload shutdown function (requires external
circuitry). A new PC board for this revision is forthcoming, however
existing PC boards and implementations are still valid.
The schematics do not show meters. The following meters should be included:
- Modulator Positive Peak power supply current (0-15 amperes)
- Modulator Output Current (0-10 or 0-15 amperes, or 2 0-10 ampere meters, one for each module of a 2 module transmitter
- Modulator Output Voltage
- Modulator Input Voltage (62 volt supply)
- Modulator Positive Peak Supply Voltage (125 volt supply) Note: A single meter can be used, if switching is provided to switch the meter between the various supply voltages
How the Class H Series Modulator Works
Refer to the Modulator
Schematic and the Schematic of the Power Supply
for a 350 to 450 watt transmitter... The modulator shown in the
schematic can be used with any of the class E RF amplifiers shown here,
operating at up to 450 watts of power. The principle is applicable to
other transmitters and designs.
The modulator is a source-follower series modulator which is
operated nearly at saturation when the transmitter is producing only a
carrier. This results in a low voltage drop across the primary
modulator transistor (Q2), and consequently low dissipation and power
loss in the device.
The low-level drive circuitry "floats" with the output of the
modulator. The op-amp U2, along with its related components and power
supply is assembled onto a small piece of copper clad board, which is
insulated from DC ground. Since the driver is floating along with the
modulator output voltage, the input to the driver must be fed in such a
way that the modulator output voltage does not affect the input. This
is accomplished by using a differential amplifier as the first input
amplifier, and referencing both the inverting and non-inverting inputs
to dc-ground reference points. The voltage between the modulator output
and DC ground will appear at both op-amp inputs equally, and will not
appear at the output of the op-amp because there is no differential
between the inputs caused by this voltage. Only a voltage difference
between the inverting and non-inverting input of U2a will appear at the
At carrier, the collector voltage of Q2 is fed through a diode from the
carrier power supply. This supply is typically between 10 and 20 volts
higher than the desired output voltage at carrier. Transistor Q1 is
operated at cutoff when no modulation is present (carrier only).
Since Q2 is a emitter-follower, the voltage appearing at the emitter
of the transistor follows base voltage. The base voltage is
set such that the output of Q2 is approximately 46-47 volts - about
15-16 volts less than the power supply voltage of 62V. The base voltage
is supplied to Q2 through the driver MOSFET, through a 4.7K resistor,
fed from a Zener diode,
which drops the power supply voltage by 12V. So, the voltage measured
at the base of Q2 with respect to ground will be around 46 volts. The
threshold voltage of the driver MOSFET is approximately 4 volts (the gate voltage
needs to be about 4 volts higher than the source voltage in order for
the MOSFET to conduct). So, the source voltage from the driver MOSFET will be 4 volts lower
than the gate voltage (which is around 50V.), resulting in between 45
and 46 volts from the source of the driver to the base of Q2. Q2 will exhibit
approximately .7 volts of drop.
With this arrangement, the carrier level is set by the power supply
voltage, since the DC output voltage of the modulator will directly
track the power supply voltage. No other adjustments are necessary to
set the carrier voltage.
The gate of Q1 is connected directly to the output of op-amp U2c
through a 12V Zener diode. The DC output voltage of op-amp U2c is 0V
DC, as measured between the floating sub-assembly and the op-amp
output. The floating sub-assembly is connected to the source of Q2
(which is the output point of the modulator). At carrier, the source of
Q2 and the modulator output is sitting at approximately 46 V above DC
ground. So, the gate of Q1 will be approximately 12V higher, or around
During the positive peak modulation cycle, the output of U2b and U2c
will increase. This will drive the gate of the Q2 driver higher (Q3), driving the
base of Q2 higher. Since the emitter voltage of Q2 will follow the base voltage, the voltage drop
across Q2 will decrease, and the Q2 emitter voltage, and the modulator
output voltage will increase. At the same time, the voltage fed to the
gate of Q1 is also increasing. When the gate voltage of Q1 reaches
approximately 66 volts with respect to DC ground, Q1 will begin to
conduct. So, the voltage drop across Q2 will drop by approximately 8V
before Q1 begins to conduct. Since the voltage no-signal voltage drop
across Q2 is approximately 16 volts, no distortion will result, because
Q1 will begin to supply more voltage to Q2 before Q2 saturates.
At all times, Q2 is the primary modulating device. Q1 simply
supplies additional drain voltage to Q2 as the output voltage
increases. It is important to note that during positive peaks, the 200V
10A diode between the collector of Q2 and the carrier power supply
is back biased. No current flows from the carrier power supply
at this time, and the supply is effectively switched out of the
The gate voltage of Q1 is fed through a diode, and therefore the
gate will only be driven when gate voltage, as referenced to
ground exceeds the source voltage. Otherwise, the diode is back-biased.
During the negative portion of the modulating waveform, the voltage
fed to the bases of Q2 drops, and Q2 acts as a normal emitter follower
series modulator. Voltage is fed to Q2 from the carrier power supply
through the 200V 10A series diode. The diode feeding the gate of Q1 is
back-biased, Q1 is completely cut off and effectively out of the
I have also designed a very simple mic
preamp/equalizer/compressor (peak limiter) to go with the class H
modulator. The schematic is Here.
The Power Supply
Schematic of the Power Supply
for the class H modulator