single stage “Zen” amplifiers

ref Link: http://www.diyaudio.com/forums/diyaudio-com-articles/214808-amp-camp-amp-1-a.html

The Design

As mentioned before, we want a simple, good sounding design. For the past twenty years or so I've been writing up single stage “Zen” amplifiers and other simple designs, and we are going to channel some of those to get what we want. Here is the very simplified circuit:

In this circuit we see a single gain element Q1, an N-channel power Mosfet. In the diagram you will see that the three pins on this device are labeled Source, Drain and Gate. The input signal comes to the Mosfet Gate pin, and the output signals appears at the Drain pin. The Source is attached to circuit Ground. This arrangement is known as “Common-Source” operation. Of the three possible ways of operating a Fet gain device, it is the one that offers both voltage and current gain.

The other two operating modes are known as Common-Drain and Common-Gate. Common-Drain has the signal input going to the Gate pin and takes the output from the Source pin. It offers current gain, but not voltage gain. Common-Gate operation has input to the Source pin and takes the output from the Drain. It offers voltage gain but not current gain.

This Common-Source Mosfet is operated single-ended Class A. This means that power goes into the circuit from one end only, and the transistor will be conducting current at all times through its operation. This current is delivered to the transistor through the Constant Current Source, which is a circuit which provided constant current, (ideally) no matter what the voltage conditions are. This constant current is shared by the Mosfet and the loudspeaker, and the input signal varies the degree to which these elements share this current, and this is what drives the loudspeaker.

We will note that Common-Source operation intrinsically inverts the phase of the amplified signal, and so we have reversed the polarity of the output terminals to account for that.

As a concept, you can buy an integrated circuit constant current source, but we are going to design our own, partly to do-it-ourselves and partly because we want to alter its character a little to improve the performance of the circuit. Here is the circuit with some details added showing the actual discrete circuit of our current source: