Impedance matching in LNA

[LakshmiJL]

HI

I am designing a LNA. My doubt is should the magnitude of the Load impedance be matched to 50ohm output termination or the real part of the load impedance be matched.Say i am using a parallel LC load. How should the matching be done to 50ohm output termination.

Kindly clarify.

 

[BigBoss]

If you use a parallel LC circuit as frequency selective circuit, it will behave approximately open circuit at the frequency of operation.All you should do is to match complex conjugate of Output Impedance of the LNA to 50 Ohm Load ( termination impedance ).

 

[Borber]

In other words if the real component of the output impedance of LNA is 50 Ohm then you can with parallel LC to 50 Ohm load conjugate complex match you load to complex output impedance of LNA either it has capacitive or inductive character. In this case you simply detune LC tank.
If real part is different from 50 Ohms then you will still have mismatch even reactive component is tuned out. You must use either low pass or high pass filter to achieve appropriate match to 50 Ohms load and you will have to check for frequency bandwidth of such solution.

 

[LakshmiJL]

thank you. but am not clear on this. i am using a cascode configuration LNA.if i use a LC load i only have imaginary part.will the output impedance of the cascode give the real part for matching?.

 

[Borber]

Output can have complex output impedance. Generally your load to LNA output is nominally 50 Ohms. If output is lets say 50+j10 then you terminate it with 50-j10 what is conjugate complex termination. LC tank parallel to 50 Ohms can be detuned to complex conjugate match to your output.