I have used transformer(even I have tried with ideal_balun available in analog library of Cadence) at the differential output and terminated with a port . I have measured output impedance of the differential output and same thing set at output port.(i.e K times where K is turns ratio.) . The problem is compare normal measurement (i.e vout/vdiff) gain is falling by almost 10 dB.....i.e S21 is around only 4 dB instead of 14 dB..
Thanks in advance....
Hi, the value of S21 will be equal to the voltage gain if the input an output impedances of your circuit arre matched (Zin=Zout). For the input, the LNA is matched to 50 Ohms, but for the output not necessary, since you want voltage gain at RF.
If you want to measure S parameters, you should add a 50 Ohms Buffer at the output of the LNA.
Dear Sir (David Córdova)
Thank you for your reply....I will try with buffer at the output......But what about noise of that buffer, I think it will impact the NF of LNA. Could you please help me ..
Hi, well the buffer will add some noise (maybe around 0.5dB at worst), but you can characterize the LNA's NF with and without buffer, since your next stage is usually a downconversion mixer and not a 50 Ohm load.
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