In the world of electricity, three types of materials are present: conductors, insulators and semiconductors. A conductor transfers electricity well. An insulator blocks it. A third class of material, a semiconductor, sometimes is an insulator, and sometimes is a conductor. A transistor, which is a semiconductor, has three parts: an emitter, a base and a collector. Electricity flows from the emitter to the collector; the base is placed in between the two. Depending on how much electricity is placed on the base, the amount of flow between the emitter to the collector is controlled. Think of a pipe, with a flow valve in the middle. The valve is the base, and the control voltage controls the handle, thus controlling the flow.
Like the valve in the pipe, transistors allow current flow only one way. A PNP transistor allows current flow from the collector to the emitter. A NPN allows current flow from the emitter to the collector. In effect, they are reversed from each other. For a push-pull amplifier to work, both NPN and PNP transistors must be used. The way to tell them apart on the wiring diagram is the direction of the arrow on the emitter touching the base. The NPN transistor arrow is "Not Pointing iN" (NPN). The PNP is "Pointing iN Positively" (PNP).
In a push-pull amplifier, also called a push-pull follower, both NPN and PNP transistors are used. A negative voltage is fed into the collector of the PNP transistor. A positive voltage is fed into the collector of the NPN transistor. Both emitters are tied together, and fed to the output. The bases are tied together, and the control voltage is fed into both bases.
Because the transistors are wired in the push-pull configuration, the control and output voltage is pushed and pulled. When one transistor is pushing a positive output voltage out, the other one is pulling a negative control voltage in. When the control voltage waveform reverses, the transistor action also reverses. At all times when one is pulling in, the other is pushing out. This action reduces the distortion of the amplifier, making for a cleaner signal at the speaker. Furthermore, because two transistors are used instead of one, neither has to work as hard.
Distortion is defined as a clean waveform at the input, but a distorted waveform at the output. Right at zero volts input, when one transistor is turning on and the other one is turning off, a slight time lag may exist. This causes a flat spot at the output wave, called "zero-point distortion." This is undesired. To eliminate this, diodes are wired in at the bases. A diode looks like an arrow on the wiring diagram. The effect of diodes is they act like buffers, so its not a hard "on" or "off," but rather a gradual transitioning of voltage from one transistor to the next. This is a popular method of overcoming zero-point distortion.