"The Sliding Challenger" - by Pat Pending

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.