The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> RF Design >> Impedance Matching in RF design -- required or not
https://designers-guide.org/forum/YaBB.pl?num=1118280194

Message started by rfzingle on Jun 8^th, 2005, 6:23pm

Title: Impedance Matching in RF design -- required or not
Post by rfzingle on Jun 8^th, 2005, 6:23pm

Hi ,
I am a newbie in RF design and a self-learner. Here is what I understand of why matching is required or not in RFIC design. Please comment on this and correct if not accurate.

Microwave power is a scarce quanity and one does not want to waste it. Hence as stated by the power transfer theorm , for maximum power transfer impedance matching is required. However it is a scarce quanity only when there is no source to amplify or provide power gain. Thanks to MOSFET.., one can get virtually infinite power gain (non-idealities like gate resistance or NQS effect apart), that one can afford not to care about power transfer on ICs. Moreover on ICs, the dimensions compared to wavelength involved are so small , that there is never a transmission line effect . Or in other words there is never a full wave travelling. Hence this microwave power cannot be dissipated through reflection (due to impedance mismatch). Thus all one cares about is voltage transfer and not power transfer. T

Thus for eg. at the interface between LNA and Mixer there is no need to do an impedance match . Infact impedance match will only hurt as needless power is dissipated in this active matching element (say 200 ohms). Howefer matching is required at the LNA and external world interface , otherwise all that microwave power would never make it to the input of LNA (even though there is MOSFET as input of the LNA).

Please let me know if that makes sense. In general the question is would be ever make sense to do an impedance match between LNA and mixer interface on an IC.

Thanks
New Member

Title: Re: Impedance Matching in RF design -- required or
Post by Frank Wiedmann on Jun 9^th, 2005, 11:34pm

In general, it does not make sense to do an impedance match on chip. The main reason for matching the impedances at the external chip interface is usually to avoid reflections which might distort the incoming signal. The transfer characteristic of an external filter might also be influenced by the input impedance of the chip.