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Design >> RF Design >> calculate the input impedance for cmos
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Message started by wwm101 on May 1^st, 2008, 7:45am

Title: calculate the input impedance for cmos
Post by wwm101 on May 1^st, 2008, 7:45am

why the follow circuit diagram which input impedance equal to 1/(gm1a+gm1b) in the radio frequency?
this problem confuse me long time .pleas help!

thank you for your attention!

Title: Re: calculate the input impedance for cmos
Post by pancho_hideboo on May 1^st, 2008, 9:30am

wwm101 wrote on May 1^st, 2008, 7:45am:

why the follow circuit diagram which input impedance equal to 1/(gm1a+gm1b) in the radio frequency?

If it is not in the radio frequency, it is very simple.

Transconductance of inverter is Gm=gm1a+gm1b.
So input admittance Yin seeing right at node X is like following.

Yin=GF*(YL+Gm)/(YL+GF)
where GF is feedback conductance for self bias, so GF=1/R, R is resistor between X and Y.
YL is load admittance at node Y.

If ZL=1/YL=infinity, Yin=Gm=gm1a+gm1b.
If ZL=1/YL=0, Yin=GF=1/R.
If GF=0, Yin=0.

So if impedance seeing right at node Y is enough high, ZL >> 1/Gm, 1/GF=R
an input impedance Zin seeing right at node X will be 1/Gm=1/(gm1a+gm1b).

For the radio frequency, this is not true.

Title: Re: calculate the input impedance for
Post by wwm101 on May 2^nd, 2008, 12:37am

TO:pancho_hideboo

thank you very much for your explain.