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https://designers-guide.org/forum/YaBB.pl Design >> RF Design >> inductive degeneration analysis https://designers-guide.org/forum/YaBB.pl?num=1224749852 Message started by aaron_do on Oct 23rd, 2008, 1:17am |
Title: inductive degeneration analysis Post by aaron_do on Oct 23rd, 2008, 1:17am Hi all, I'm trying to analyze inductive degeneration (LNA) using basic feedback analysis. Assume i break the loop at the source terminal of the transistor. So the open-loop transconductance is simply gm, while the feedback factor should be sLs making the loop gain gmsLs. Therefore, the overall transconductance is, Gclosed = gm/(1+gmsLs) which seems to be correct. For the input impedance, it should be something like, Zin = (1/sCgs)*(1+gmsLs) =1/sCgs + ωTLs However, the actual solution for Zin should be, Zin = 1/sCgs + ωTLs + sLs I have a feeling i've gone wrong somewhere by not considering the loading, but i'm a bit confused how. Any help is appreciated. thanks, Aaron BTW i have assumed that the circuit is fed by a voltage source, not a 50 ohm source... |
Title: Re: inductive degeneration analysis Post by buddypoor on Oct 23rd, 2008, 3:17am aaron_do wrote on Oct 23rd, 2008, 1:17am:
I did not recalculate all the formulas, however, I think the overall transconductance is not correct as you did not take Cgs into consideration. If you like to consider Cgs during Zin calculation you should do this also for the gain resp. overall transconductance as a part of the current through the source terminal originates from the current through Cgs. May be that is the cause of the discrepancy. Added some time later: In the mean time I have thougt a little about the problem - and I think there is another reason for the discrepancy (but it is connected with the above mentioned arguments): The input impedance is increased by the same factor which decreases the gain (resp. transconductance) due to feedback only in the case that there is a CLEAN voltage feedback without any disturbance. But in your case, the element Cgs disturbs this feedback - and therefore the situation is a bit more complicated and we cannot expect that there is a common factor (1+gmwLs) for the main parameters. |
Title: Re: inductive degeneration analysis Post by RFICDUDE on Oct 26th, 2008, 7:07pm I think it is simply because you end up with the series impedance of the element plus a feedback term. The feedback term is what produces ωTLs (combination of sCgs and gm transforms the inductive reactance into a real resistance) while the impedance simply do to the inductor in series with the input is sLs. A similar thing happens with series series feedback for common emitter amplifiers with resistive degeneration. |
Title: Re: inductive degeneration analysis Post by aaron_do on Oct 26th, 2008, 7:43pm Hi, thanks for all the help. I think this point by buddypoor may have been the main thing... Quote:
Anyway i got the right answer using blackman's formula...obviously kvl and kcl works too... cheers, Aaron |
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