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Using a negative power supply would allows us to scale up input and feedback resistors’ values. With a bipolar power supply, the amplifier now sports a 10k input impedance, which is not only easier to drive, but also allows the input coupling capacitor to be reduced to 2µF, a film capacitor rather than an electrolytic. An added feature of both inductively-loaded amplifiers is that they come pre-centered and not need to be user adjusted.

In the original Pass Zen amplifier, the device MOSFET worked as a constant current source that forced a predetermined amount of current through he bottom MOSFET, whose input bias voltage had to be adjusted to center the output at half of the rail voltage for the largest symmetrical voltage swings (what a drag). But as an ideal inductor displaces no voltage (and real inductors only displace a minute voltage) the amplifier comes pre-centered. (An alternative might be to use only N-Channel MOSFETs and reconfigure the Zen so that the device MOSFET still functions as a constant current source, but a source that auto-centers the output. The amplifier to the right shows how to make the convergence into the TCJ Zen amplifier.)

Yet I am still troubled by the need for a large-valued input coupling capacitor and the low input impedance. By using a AA battery, we solve both problems at once. Returning to the inductively-loaded amplifier as an example (the same technique can be used with the previous circuit), we see DC coupling and a higher input impedance. Still, 10k is far too low an impedance for many tube line amplifiers.

So why not scale the input and feedback resistors up any further? MOSFETs have a great deal of gate-to-source capacitance, which effectively shorts out the feedback resistor at high frequencies. In other words, the greater the feedback resistor’s value, the lower the high-frequency cutoff frequency.

Pause and reflect: if we are using a tube line amplifier that can cleanly swing 30 volts, why not exploit that virtue rather than fight the liability of a high output impedance? The circuit at the right is configured as an unity-gain source follower, which offers a high-input impedance and low output impedance and low distortion, but at the cost of a large input voltage swing. This amplifier/buffer uses all of the MOSFET’s transconductance to offer the low output impedance and low distortion, but no gain. With the line amplifier providing all the voltage gain, the amplifier/buffer can concentrate on accurately delivering voltage and current into the load.

As a test to see if you are following along, does the amplifier in the next schematic function as a buffer (a source follower) or as an amplifier (a grounded-source amplifier)? Is there a global feedback loop? Why aren’t the output coupling capacitors connected to the MOSFET’s source?

The answer is that the circuit is an amplifier, an amplifier that uses two feedback resistors, the 1M connecting to the positive terminal of the power supply and the 143k resistor that connects to the other end of the inductor. Notice how moving the reference point made all the difference.

Conclusion

We have seen that a simple amplifier can easily become complex and that a simple amplifier can be novel. We also saw how a simple amplifier made increased demands on the associated equipment.

Well, I will stop here at single-ended designs, as the long article is the enemy of the published. Next time, I will cover simple push-pull amplifiers. If you have any comments or questions, be sure to send them to me at editor@glass-ware.com.

//JRB