Page 9

2003 

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Copyright © 2003 John Broskie    All Rights Reserved

Generic circlotron push-pull class-AB amplifier with the signal reference at the center of the output

As far as the triodes are concerned, they are in the exact same circuit, as nothing has changed, the same voltages and the same currents.

For example, imagine placing a 2-volt battery across the outputs of these two amplifier topologies. In the first amplifier, the top triode’s cathode will be forced positive by 1 volt and the power supply’s center point will be forced negative by 1 volt, which will in turn force the bottom triode’s cathode negative by 1 volt. The result is that since none of the grids have shifted in voltage, the top triode sees a 1-volt more negative grid voltage and it conducts less; the bottom triode, a 1-volt more positive grid voltage and it conducts more.

This then pulls the bottom triode’s plate down and top triode’s plate up. In other words, the tubes strive to correct the battery’s voltage across the output, their transconductance powering their efforts. In the circlotron amplifier, the same battery across the output causes the exact same result, as one triode conducts less while the second conducts more, striving to nullify the battery’s voltage.

Notice that in both cases that each triode only saw half of the battery voltage (1 volt) and in anti-phase to each other, which makes sense as the reference falls exactly half way between the output terminals of the amplifiers. Notice also that in both cases each triode saw the entire battery voltage (2 volts) superimposed on its cathode-to-plate voltage, negatively on the first triode, positively on the second triode.

In both amplifiers, the battery experiences a current flow when hooked up across the output terminals and the amount of current that flows through the battery is directly related to the output impedance of the amplifiers. For example, if 1 ampere of current flowed, then the amplifier has an output impedance of 2 ohms, as I = V/R, or in terms of R, R = V/I.

Let’s pause and consider this: as both amplifier topologies share identical characteristics, has just adding two resistors and shifting the ground magically transformed the vanilla, push-pull, very lean running, class-AB, totem-pole amplifier into a single-ended, class-A, magic-imbued amplifier?

2-volt battery placed across the outputs of the push pull amplifiers to test the amplifier’s output impedance