Load has to be larger than output impedance - why? Matching?

[arefeidi]

Hi,
I've got a receiver which mixes down a 1GHz signal to ≈ 100kHz. The receiver has two outputs (differentially, 180° phase shift) which both have an output impedance of 65Ω. In the datasheet of the receiver it says "The loading resistance on each output (single-ended) should be larger than 300Ω to assure full gain".

Now I've always learned that the output impedance has to match the loading resistance to not get reflexions and to get maximum power at the loading resistance. So why it's all different here, why the loading resistance has to be much larger than my output impedance?


Thanks for all answers!

 

[A.Anand Srinivasan]

Re: Load has to be larger than output impedance - why? Match

what are you mentioning is power match... in that case the output impedance and load have to be matched so that maximum power is delivered to the load.... just consider output impedance and load being as arms of voltage dividers and then find maximum power delivered by differentiation and other stuff you'll know.....

but for the case of voltage gain
you have to see the output impedance and load as arms of voltage divider and hence if the load is larger than the output impedance then it gets more voltage across it....

 

[arefeidi]

Re: Load has to be larger than output impedance - why? Match

Thanks for your help, I think I got it now!

For low frequencies and short cables reflexions etc aren't really a problem, so no power matching is needed. Didn't think about that - probably I did two much RF stuff the last months.