Totem Pole

Page 5

2003

In the next circuit, see the same basic topology varied slightly (or hugely, but familiarity has jaded us too much to see it so). The signal reference point has been shifted from the power supply’s midpoint to the bottom triode’s cathode. Few would regard this change as being worthy of controversy, as the amplifier functions identically to the previous version. (Although, there would be some controversy surrounding the use of a coupling capacitor on the output.)

Understand, however, that there is no controversy about how these variations on a the basic circuit work, no magic, no physics defying stunts, just pure vanilla circuitry. They are purely push pull designs that can be biased to work in class-A, class-AB, or class-B, but are usually biased in a lean class-AB, because of the high plate voltages. No one claims that they are, in fact, two single-ended amplifiers in parallel (or single-ended amplifiers in series) or that they must be, by necessity, class-A amplifiers.

Now, let’s make things a bit more interesting: let’s move the ground reference once again; this time, to the top triode’s cathode. In this variation shown below, once again the same cathode follower functioning has been retained, thus the larger input signal swing for the bottom triode than for the top triode.

Generic one-tube-on-top-of-the-other push-pull class-AB amplifier with the signal reference shifted to the bottom triode’s cathode and with a unipolar power supply

Wait a minute, how can this be? Since the top triode’s cathode is now quite literally grounded, how can it function as a cathode follower? If the driver stage returns all of the top triode’s voltage gain to its grid in anti-phase, then the triode functions as a 100% negative degeneration amplifier, whose most common realization just happens to be the classic cathode follower topology, but not necessarily so. Appearances are not as important as voltages and current flow.

Once again, the tube doesn’t know that it is in something called a “cathode follower” or a “cascode” or a “SRPP;” it only “knows” that its cathode-to-plate voltage is such and such an amount and that its cathode-to-grid voltage is such and such an amount. And in response to the changes in cathode-to-grid and cathode-to-plate voltages, it conducts either more current or less current. This strict determinism disheartens and frightens those who believe in freedom of will for electrons, but it is essential to understanding how circuits work.

Here is an analogy that might help: when Copernicus shifted our solar system’s reference from Earth to the Sun, the planets were oblivious to the shift and they blindly continued along their paths as if nothing had happened. We were not so lucky, as least one man was burned at the stake as a result.

Generic one-tube-on-top-of-the-other push-pull class-AB amplifier with the signal reference shifted to the other side