SiGe LNA input impedance

[haisuki]

Dear all,

the matter is to obtain analitically the input inpedance of a degenerated common-emitter stage of a SiGe transistor.

I have seen many papers, for CMOS, with the explanation of the benefits of a source inductor in a common-source configuration obtained from the small signal equivalent model of the transistor. Among others, 50Ω input impedance can be obtained more easily and gain is improved by the quality factor of the input stage.

But, for the case of a bipolar transistor (a SiGe HBT in my case), there is a shunt resistance (Rbe) in the small signal equivalent model, that does not exist in the CMOS transistor. If Rbe is added, calculations became more complex. I think the benefits of the inductive emitter degeneration can be shown, however I do not see clearly whether the quality factor increases the gain.

I do not want to have a longer post, but feel free to ask anything, I will be glad to explain further this matter.

Thank you in advance

 

[khouly]

the emiter degeneration inductor in SiGe will make the noise matching more easily , it will increase the input impedance of LNA , and by this u can adjust the Roptimum to get the NFmin of the transistor
about Q , if u increase the Q the series R of of the L will be min , so the gain of the LNA will increase , but also be aware that inductor degeneration will decrease the total gain of the transistor alone , coz this coil make a feedback loop ,which decrease the gain

khouly