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Design >> RF Design >> max power transfer theorem
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Message started by raja.cedt on Nov 6^th, 2008, 3:05am

Title: max power transfer theorem
Post by raja.cedt on Nov 6^th, 2008, 3:05am

hi all,
i am little bit confused with max power transfer theorem and max efficiency,where to use which one.for example in amplifiers max efficiency theorem,where as in RF i saw max power.Can any body please explain this in detail.
Thanks .

Title: Re: max power transfer theorem
Post by Peruzzi on Nov 6^th, 2008, 10:52am

Raja.cedit,

Regarding maximum power, you can convince yourself of (as opposed to proving) the theorem's validity in a few minutes by drawing a circuit with voltage source, source resistance and grounded load resistance. Keep in mind that we're talking about power, not just voltage or current. Try both DC and AC.

Efficiency can be tricky, because efficiency has more than one definition. Nothing really difficult. Dust off your old circuits and systems textbook and review it. You can probably find what you need in wikepedia too.

Bob

Title: Re: max power transfer theorem
Post by RFICDUDE on Nov 15^th, 2008, 6:42am

Yes, knowing where to apply each of these is confusing sometimes.

Here are the main differences
Conjugate matching solves the problem of maximum power transfer between an AC source and an AC load at the same frequency. It makes no assumptions or conditions regarding power efficiency.

Power efficiency is measured by the AC power delivered to the load in comparison to the DC power dissipated by the amplifier circuit. The conditions here are completely different than conjugate matching. The only common point is power delivered to the load.

You would think that delivering the maximum available power to the load ought to be related to the condition for maximum efficiency because, well, the circuit is delivering maximum available power. But this is deceptive because by its definition conjugate matching results in half of the AC power being dissipated within the source itself. Therefore the DC power has to be high enough to supply the both the AC power dissipated across the internal resistance of the amplifier and the load.

Steve Cripps has a somewhat famous conference paper which succinctly shows, in part, why conjugate matching is less power efficient than (what he calls) power matching. He also describes this in one of his books.

Steve C. Cripps, RF Power Amplifiers for Wireless Communications 2nd Edition, Artech House, 2006.

S. Cripps, “A Method for the Prediction of Load-Pull Power Contours in GaAs MESFETs,” Proc. IEEE Intl. Microwave Symp., MTT-S, 1983, pp. 221-223.