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Tuesday, July 10, 2012

Maximum Power Transfer Theorem

The Maximum Power Transfer Theorem is not so much a means of analysis as it is an aid to system design. Simply stated, the maximum amount of power will be dissipated by a load resistance when that load resistance is  equal to the Thevenin/Norton  resistance  of  the network   supplying   the  power. If  the  load  resistance  is  lower  or  higher  than  the Thevenin/Norton resistance of the source network, its dissipated power will  be less than maximum.

                     
Taking our Thevenin equivalent example circuit, the Maximum Power Transfer Theorem tells us that the load resistance resulting in greatest power dissipation is equal in value to the Thevenin resistance

This is essentially what is aimed for in radio transmitter design , where  the  antenna or transmission line “impedance” is  matched  to  final power amplifier    “impedance” for maximum radio frequency power output.  Impedance,  the overall opposition to AC and DC current, is very similar to resistance, and must be equal between source and load for the greatest amount of power to be transferred to the load. A load impedance that is too high will result in low power output. A load impedance that is too low will not only result in low power output, but possibly overheating of the amplifier due to the power dissipated in its internal (Thevenin or Norton) impedance.

The Maximum Power Transfer Theorem is not: Maximum power transfer does not coincide with maximum efficiency. Application of The Maximum Power Transfer theorem to AC power distribution will not result in maximum or even high efficiency. The goal of high efficiency is more important for AC power distribution, which dictates a relatively low generator impedance compared to load impedance.

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