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Does PA output impedance need to match to 50Ω?

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yantom

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in generally, the input and output of chip should be matched to 50Ω. But if output impedance of PA matches to 50Ω, the efficiency is smaller than 50%. In reality the efficiency can go as high as 90%. So i want to know whether the output needs to match to 50Ω. This problem has confused me for a long time. thanks
 

PaulHolland

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Simply read this: http://en.wikipedia.org/wiki/Maximum_power_theorem and you will master it :).

The error you make is that the internal resistance is not a resistance that is truely there. Its part of the system !. So if you short the system the maximum current is A. and if the system is open the maximum voltage of the system is B. Than we say. R = B/A. But this means that we represent the circuit as a perfect voltage source with a resistor in series... Represent, so its not real !. Get it ?. Maximum power transfer you if the A and B in my example are both the highest you can get and this is only true when Rs = RL in my example. So if your internal resistance is 5 Ohm, maximum power transfer happens when RL (Load resistance) is also 5 Ohm.

Now people will say, but my audio amplifier does not have a 8 or 4 Ohm source resistance and the Load resistance is 4 or 8 Ohm. True. But they forget that a loudspeaker is a current device, the more current the larger the movement of the cone. So How do we get maximum current output, see my example ?. When Source resistance is smallest, since the short circuit current will be maximum than.

Same is true for Motor drivers.

With RF amplifiers we also match the source resistance to 50 Ohm since 50Ohm is the standard impedance of coaxial cable. To get minimum reflection we need to have a source/load impedance of 50Ohm. So we convert the low or high source impedance to 50 Ohm to get maximum POWER transfer from our device to the load.
 

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