.
Combination class H modulator - power supply by Bob, K1KBW.
This implementation uses 10 IRFP260N MOSFETs, 5 in the each leg of the modulator.
Modulators and Audio
In theory, any type of high level modulator can be used with a class
E RF amplifier to form a complete AM transmitter. The modulator can be
transformer, series or shunt coupled. However, the best type of
modulator to use is some type of series modulator. Such a modulator can
use any technology - class A analog, high efficiency analog, or digital
(Pulse Width), as long as the result is modulated DC applied to the RF
amplifier. Two types of modulators are described here: A high
efficiency analog modulator using class H and a Pulse Width modulator (the
most efficient of the modulators)
Modulator types and advantages of each
Modulators fall into two general classes - Analog and Digital (pulse width).
Both have advantages.
The analog modulators will operate over a wider range
of load currents and resistances, and are generally fairly straight-forward in design.
The audio quality is generally superb - smooth frequency response and low distortion.
Because of these characteristics, along with the
relative simplicity of operation, these types of modulators have been
extensively used by amateurs building class E transmitters.
The primary disadvantages of analog modulators are the size and weight and heat production.
Analog modulators are much larger than comparable digital (pulse width) modulators, and
have significantly larger power supply requirements.
Digital (pulse width) modulators are the most efficient (up to 95% efficiency is possible),
and can be made smaller, and require smaller and simpler power
supplies than comparable analog modulators. Pulse width modulators are designed
to work into a particular load resistance - the load in this case being the
class E RF amplifier. The class E amplifier must be operated within
certain parameters to properly match the design impedance of the modulator.
Digital modulators are designed with significant
filtering, by the nature of how they work, and therefore can offer better
control over the transmitted bandwidth than analog modulators. A properly
designed and implemented pulse width modulator can produce broadcast quality
audio, and the system is widely used in modern AM broadcast transmitters.
A Class H modulator is an 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.
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
state designs.
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.
How Class H Modulators Work
A class H modulator starts with a source-follower series modulator (Q2)
operated very close to the saturation point most of the time, and a 2nd device (Q1) that
supplies additional voltage to Q2 when needed.
The collector voltage of Q2 is fed through a diode from the
50V carrier power supply. The supply voltage is usually somewhat
higher than the desired output voltage at carrier. Transistor Q1 is
operated at cutoff when no modulation is present (carrier only).
Since Q2 is an 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 around 40 volts - about
10 volts less than the power supply voltage of 50V.
During the negative portion of the modulating waveform, the voltage
fed to the base of Q2 is driven lower, and Q2 acts as a normal emitter follower
series modulator. Voltage is fed to Q2 from the carrier power supply through
diode D1.
Q1 is not conducting during the negative portion of the waveform,
and is effectively out of the
circuit at that time.
During the positive peak modulation cycle, when the modulator is required
to deliver more voltage, the gate of Q1 and the base of
Q2 are both driven 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, and Q1 will begin to conduct.
Q1 will begin to supply more voltage to Q2 before Q2 saturates and
causes distortion. The output voltage will continue to rise, up to the positive peak power
supply voltage of 100 volts, as long as the base of Q2 and the gate of Q1
continue to be driven higher.
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, diode D1
connected between 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
circuit.
A complete modulator using class H is featured elsewhere, in the Construction
Projects section of this document.
Pulse Width Modulators
A pulse width modulator is essentially a switching power supply, where the
output voltage of the supply can be controlled by an external input. In this
case, we feed audio into that input, and control the output of the switching
supply with the audio we're supplying. Audio amplifiers using pulse width
modulation are becoming quite common, particularly for high power amplifiers.
Brief discussion on how Pulse Width Modulators operate
Pulse width modulators operate by varying the duty cycle - the "on time"
as compared to the "off time " of a switching (square wave) waveform.
This switching waveform is produced using relatively simple low level circuits,
and is amplified using switching (either saturated or cutoff) amplifier stages,
to the desired output voltage.
The output of this amplifier is then filtered, removing the switching frequency.
After filtering, the output is the average voltage of the switching waveform.
By controlling the switching waveform on-time as compared to the off time, we
can control the output voltage, after filtering, of the amplifier. Because
the amplifier stages in a pulse width modulator are operated either at cutoff
or saturation (this is called switch mode, or class D), such modulators are
typically very efficient. 95% efficiency is achievable with practical
circuits.
The advent of some very good Pulse Width Modulator ICs over the past few years has significantly
simplified the design and construction of these types of modulators. Excellent
results can be achieved with comparatively few components. The filter network
components are fairly small, at least up to 400 or 500 watts, and it is possible
to build a 400 watt pulse width modulator that you can hold on one hand.
More Information about Pulse Width Modulators
A complete explanation of Pulse Width Modulators, and how the PWM
Modulator works can be found in this Solid State PWM paper.
There is also a similar writeup on Vacuum Tube PWM.
