Push-Pull Amplifiers

Follower and the Cascode look nearly identical. But if we look at the current phases within the circuits, we see that the Cascode has but one current phase and that both tubes will always share the same amount of current flow as they are in series with each other. The White Cathode Follower has two current phases and only the quiescent current flows through a single current path. An AC signal provokes one tube into greater conduction and the other into lesser, as the AC current phases are in opposition with each other.

Wiring the circuit up to a bipolar power supply and connecting the output to ground helps make the point clearer. If a positive input signal is large enough, the bottom tube will cease to conduct altogether as the voltage developed across the plate resistor will be great enough to drive the bottom tube's grid so negative relative to its cathode that it no current can flow from cathode to plate.

Figure 1. Transformer coupled Push-Pull Amplifier

  Conversely, if a negative input signal is large enough, the top tube will cease to conduct altogether as its grid will be so negative relative to its cathode that no current can flow. The bottom tube on the other hand, will see a large positive voltage on its grid from the collapsing of the voltage across the plate resistor, which will provoke a large increase in current flowing through the tube.
  Now if we wire a load resistor in series with the output and ground, we will see how each tube delivers current into the load and how the current flow through the tubes will be in anti-phase with each other in the presence of AC signal.

Figure 2. Bridge Push-Pull Amplifier

  The same analysis holds true for the SRPP. We start by redrawing the circuit for use with the bipolar power supply and ground the out-put. A large positive input signal will cause the bottom tube to increase greatly its conduction of current, which  will pull its plate voltage down because of the greater voltage developed across the 1k resistor, which in turn,

Figure 3. SRPP
Push-Pull Amplifier

Figure 4. White CF
Push-Pull Buffer

  All of the above circuits are examples of push-pull amplifiers. In all cases Class AB operation is possible, as each triode can be turned off for some portion of the output waveform. Having at least two tubes is a necessary condition for push-pull operation, but not a sufficient one, as the Cascode amplifier uses two tubes, but is still entirely single-ended in operation. If we squint, the White Cathode

will turn off the top triode as it forces its grid too negative for any current to flow. Conversely, a large a negative input signal will eliminate the bottom tube's conduction altogether,  which will provoke a great increase in current

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