The following was offered by my friend Pat Pending as an exercise to build an amplifier with minimum parts using the EMOS. 3 watts from one active device could be a crystal radio builders delight. Kiss the LM386 goodbye? Now Pat's circuit:
I've always been interested in vintage electronics, and the methods that the engineers and technicians used to overcome design problems in bygone days.
One such example is from
the early transistor era. Transistors then were very expensive, and
the desire to make savings with production costs were sought, class B
push pull amplifiers with their two output transistors were costly to
produce, and the class A alternative suffered from high power
consumption, not only that, they also consumed the same power
regardless of the required audio output, because the quiescent current
in the output device had to be set so as to handle the maximum
without distortion. The fixed bias class A output stage has a maximum
theoretical efficiency of 50% falling to 0% at minimum output.
Enter the sliding bias
amplifier. The principle is to provide "just enough" drive
to the output device to allow it to handle the incoming signal level
without distortion, in simple arrangements there are two main methods
that were used, feedback biasing, where a part of the amplifier
output is rectified and used to provide output drive, and feed
forwards biasing, where part of the drive signal is used to provide
DC bias, the later method is the one that has the potential to create
less distortion in the output as the increase in drive bias is
available the instant the signal increases, well, in theory at least.
Sir Douglas Hall K.C.M.G.,
M.A.(Oxon), devised such a circuit that was published in the "Radio
Constructor" magazine in August 1970 entitled "The Sliding
Challenger", the circuit contained three bipolar transistors,
and had a power of 250mW. His intention was that it would be used by
the hobbyist as an output stage in home brew battery powered radios as
an alternative to the common class B topology
One of the problems that
the circuit suffers is that as the output device drive is provided by
the applied signal, and therefore requires an extra stage of
amplification, to provide enough drive. After a discussion about
eMOS-FET transistors I decided to see if the high input impedance of
the mos transistors could be the answer, allowing the construction of
an effective one transistor audio stage.
The circuit suggested
provides the adjustable voltage bias to the gate of the device to VTh
making it operate in saturation mode, the bias is applied through a
diode to produce a clamp that sits the drive signal atop the fixed
voltage, theoretically the device never running out of drive. If the
circuit is built and the drive is insufficient, try different values
for the capacitor/series resistor that are between drain and gate.The
only setting up proceedure is to measure the drain current and adjust
the bias so that e-MOS passes 10-15 mA at no signal,
the zener diode voltage is
chosen to be just above VTh for the e-MOS used, the transformer
version had a 10:1 impedance ratio (3.16:1 turns ratio). If its
preferred to direct couple the speaker, a higher resistance coil is
needed as per the second diagram, and at higher powers it must be
remembered that there will be a DC displacement of the speaker cone,
so the handling capacity must be halved to prevent damage. The
transistor used in the example was a T03 IRF450 that was to hand, (a
very expensive and high power device!) and in the transformer version
produced 3W approximately at 10% THD with an input signal of
2.5VRMS,and supply of 12volts.
No one would dare suggest
that this technology should be revived for use in the modern age but
it is an interesting experiment none the less.
