Split-Load Unity Gain Circuits
This topology admits two variations.
The first is marked by the cathode side connection, where the output
directly (or, somewhat indirectly, via a coupling capacitor or a
zener or a voltage regulator) couples to the other side of the phase
splitter's cathode resistor. Thus, any perturbation at the output is
relayed to the phase splitter's plate resistor. How? Since the
cathode resistor sees a fixed voltage at its cathode end, the
perturbation at the amplifier's output provokes an increase or
decrease of current flowing through the cathode resistor. This
increase or decrease of current finds only one current path to the B+
connection: the
path through the tube to the plate resistor. Thus any perturbation
at the output is relayed to the bottom output device's input, which
then causes either a increase or decrease in the bottom device's
current conduction to counter that perturbation.

Totem-Pole
Output Stage

PSRR
Enhancement

In all the push-pull topologies, the goal is
the same: to provide an equal drive signal for the output devices.
This goal is important, as it ensures a low distortion output signal
by forcing each output device to work equally into the load
impedance, a task required for low distortion operation. But even
when we have labored to ensure an equal drive signal, we may still
find the output signal tainted with noise from the power
supply.
Don't all push-pull output stages
reject the power supply noise from their output? Some come close,
but not all push-pull output stages see a balanced load impedance or
an equal share of power supply noise or are run in true Class-A mode. This last point is almost never mentioned, but should be,
as the push-pull's two output devices must be conducting current in
order for the noise to drop out of the output signal.
While at idle, both output devices of a Class-AB amplifier
certainly do draw some current, so the cancellation of power supply
noise is great. But once any one device stops conducting, it can no
longer work to cancel the noise. Unfortunately, as amplifiers are
only tested for noise at idle, the real dynamic noise rejection
figure is never measured.
Therefore do not
conclude that if an amplifier is push-pull, it must already be noise
rejection optimized. Thus, our added goal is to understand the
transmission mechanisms used by power supply noise to make its way
to the output.
The techniques outlined in this
article are also applicable to pure tube OTL amplifiers or hybrid
amplifiers. As a consequence, last issue's use of a schematic symbol
made up of a box with a capital "N" in it will be retained to
illustrate the universality of the techniques. Once again, this
symbol denotes either a triode, or a pentode, or a N-channel MOSFET,
or a NPN transistor. All these devices are functionally identical in
that they can be used as output devices in the six basic output
topologies.

Split-load,
cathode-referenced, unity-gain

output
stage

The second topological variation is
marked by a plate side connection, where the output directly (or
indirectly via a coupling capacitor or a zener or a voltage
regulator) couples to the top of the phase splitter's plate
resistor.

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