A paralleled amplifier configuration uses multiple amplifiers in parallel, i.e., two or more amplifiers operating in-phase into a common load.
In this mode the available output CURRENT is doubled but the output voltage remains the same. The output impedance of the pair is now halved.
The image shows two identical amplifiers A1 and A2 connected in parallel configuration. This configuration is often used when a single amplifier is incapable of being operated into a low impedance load or dissipation per amplifier is to be reduced without increasing the load impedance or reducing power delivered to the load. For example, if two identical amplifiers (each rated for operation into 4 Ω) are paralleled into a 4 Ω load, each amplifier sees an equivalent of 8 Ω since the output current is now shared by both amplifiers — each amplifier supplies half the load current, and the dissipation per amplifier is halved. This configuration (ideally or theoretically) requires each amplifier to be exactly identical to the other(s), or they will appear as loads to each other. Practically, each amplifier must satisfy the following:
- Each amplifier must have as little output DC offset as possible (ideally zero offset) at no signal, otherwise the amplifier with the higher offset will try to drive current into the one with lesser offset thereby increasing dissipation. Equal offsets are also not acceptable since this will cause unwanted current (and dissipation) in the load. These are taken care of by adding an offset nulling circuit to each amplifier.
- The gains of the amplifiers must be as closely matched as possible so that the outputs don't try to drive each other when signal is present.
In addition, small resistors (much less than the load impedance, not shown in the schematic) are added in series with each amplifier's output to enable proper current sharing between the amplifiers. These resistances are necessary, without them the amplifiers will in practice fight each other and overheat.
Another method of parallelling amplifiers is to use current drive. With this approach the close matching and resistances are not needed.
Read more about this topic: Bridged And Paralleled Amplifiers