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Cathode-Follower Totem-Pole Amplifier

This is the classic totem-pole, push-pull output stage. From only a casual inspection, the two output tubes seem to be working in different ways, as the top tube seems to function as a cathode follower (because the load attaches at the cathode), while the bottom tube seems to work as a grounded-cathode amplifier (because the load attaches at the plate). Appearances, however, deceive. Initial impressions to the contrary, in this circuit both top and bottom output tubes function as cathode followers, delivering no voltage gain and offering a low output impedance and distortion figure. Both top and bottom tubes experience 100% degeneration of their input signal, as the top tube's signal reference is ground, while the bottom tube's reference is the amplifier's output.

For example, a quick-going voltage pulse applied to the output, forcing the output one volt positive, will produce a countervailing decrease in conduction from the top tube and an equal but countervailing increase in conduction from the bottom tube, resulting in a pulling down of the output voltage. (The top tube's cathode will be one volt more positive in relation to its grid-effectively making the grid one volt more negative-and the bottom tube's grid will be one volt more positive in relation to its cathode when undisturbed.) Similarly, forcing the output one volt negative will produce opposite results, which will equally counter the negative-going disturbance at the output by pulling up the output voltage.

 

In this amplifier, the key feature is the differing voltage references: ground and output. Viewed from the perspective of ground, the input-signal swing that the top triode sees is seemingly greater than that seen by the bottom triode. The top triode's cathode must follow (in phase) the output signal, which means that its actual input signal (grid to cathode) is equal to its input signal minus the output signal.  On the other hand, the bottom triode's cathode is effectively grounded through the power-supply to ground, which means that its grid also sees the same input signal as the top tube.

 

In other words, both tubes see the same grid-to-cathode voltage swing, equal in magnitude and bandwidth, differing only in phase. For example, a positive-three-volt pulse applied to the top and (in anti-phase) to the bottom grids might induce the output to climb one volt positive relative to ground. This would mean that the grid-to-cathode voltage pulse the top tube sees would equal +2 volts, as 3 - 1 equals 2; whereas the grid-to-cathode voltage pulse the bottom tube sees would equal -2 volts, as -3 + 1 equals -2.

 

This cathode-follower functioning reduces both the output impedance and distortion, but offers no gain, which in turn will make greater demands on the driver stage or phase splitter. In contrast, with the grounded-cathode topology, things are reversed in that both top and bottom output tubes function as grounded-cathode amplifiers. The top tube's signal reference is the output, not ground, while the bottom tube's reference is ground, not the output.

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