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Intro:
Welcome back to the Zen Amp Variations. This is part 2 of many parts in which we explore some of the ways to make a very simple audio amplifier. In this and parts 3 and 4 we will embellish upon the original Zen amplifier circuit, improving the performance and creating the Penultimate Zen Amp.
You may recall that the Zen Amp is a single MOSFET transistor operated in what is known as Common Source mode in which the input signal is fed to the Gate pin, the Source pin is grounded, and we take the output signal off the Drain. In order to get this arrangement to work, we have to provide this transistor with a current source, which is just what its name implies. It is a source of current which provides the power for the gain device. In the case of the Zen amp, the current sources acts as a mediator between the positive voltage supply and the gain transistor, feeding the right amount of current into the circuit to provide the optimum conditions for the device.
The current source can be made as simple as a resistor (or a light bulb), but it is generally advantageous to make it out of something more complex, since using a resistor results in about 8% efficiency or so, and this being power circuit, wasting this much power gets costly and is socially incorrect. Just ask anyone who has built the Son of Zen, and they'll tell you that burning 600 watts to get 50 watts output is a bit over the edge.
In this article we will recap the operation of the original current source for the Zen, introduce an alternative current source, and then pull a trick out of the hat for a new source of current with a negative and occasionally imaginary source impedance.
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