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#11
March 21st 08, 01:22 AM
posted to rec.radio.amateur.antenna,sci.electronics.design,sci.electronics.basics
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Do receiver antennas need matching or not?
On Mar 17, 4:06*am, Jeroen Belleman wrote:
Mark wrote: Note: *For many LNA designs, the best signal/noise ratio occurs at an impedance that is close to, but not really, a perfect conjugate match. * The signal is coupled to the amplifier best at the conjugate match impedance, but sometimes the noise is enhanced even more. That brings up an intersting question I never did get a good answer to... It is my assertion that an LNA that is physically at room temperature (290K) can have a noise figure no better than 3 dB (i.e. its effective noise temperature is 290K) *IF it is also conjugatly matched i.e. looks like 50 Ohms. Yes you can make the noise figure better than 3 dB, but then you must either cool the device or MISMATCH it to the line. I make amplifiers with 50 Ohms input impedance and 300pV/rtHz input- referred noise. By your assertion, that should have been impossible. The trick is that the input impedance obtained by feedback: The cooled resistor trick. It works. Jeroen Belleman- Hide quoted text - - Show quoted text - good point thanks Mark |
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