# Impedence matching

#### Beautucker

[SOLVED] impedence matching

I am having some trouble deriving a proof that the max power deliveredto the load is arrived at when load resistance = output resistance. Can anyone help?

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#### Guybrush Threepwood

ok, I think it was going like this....

the source has a voltage of Us, and an internal resistance of Rs. The load has a resistance of Rl.
the voltage on the load is Ul = Us(Rl/(Rs+Rl))
the power delivered to the load is P = Ul2/Rl = (Us2*Rl)/(Rs+Rl)2
if you want to know the maximum power delivered to the load you have to do dP/dRl = 0 and that yields (if I did it correctly)
dp/dRl = (Us2*(Rs-Rl))/(Rs+Rl)3
so for this to be 0 means that Rs = Rl

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