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Design >> RF Design >> Value of DC block capacitor in LNA
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Message started by engrvip on Jun 19^th, 2019, 10:17pm

Title: Value of DC block capacitor in LNA
Post by engrvip on Jun 19^th, 2019, 10:17pm

Hi

I am designing a 2GHz common gate CMOS lna and need to add DC block capacitor at its input i.e. source of input MOS.

I have come to know that value of this cap should be approx. 2Ohm to avoid adding high impedance in signal path at desired frequency which if calculated using formula 2-Ω= 1/ωC at 2GHz; value for dc block cap comes out to be 39.8pF.

Is this the right way to calculate value for DC block cap in LNA rfic ? What if I used smaller value say 5 or 10pF. Will it disturb my input matching since I have dc biased input CG MOS such that it gives gm=20mS resulting in 50-Ohm impedance at its source and LNA input.

Regards
Vipul

Title: Re: Value of DC block capacitor in LNA
Post by Horror Vacui on Jul 24^th, 2019, 1:21am

The DC block is in series with your input, thus its impedance adds to the input impedance of your circuit w/o the cap.

Do I miss something? It seems way too simple.