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Biasing with parallel (series LC) at input

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jesuschrist

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Hi.İ've been working on LNA design but I needed to connect one series, one parallel capacitance at the input of transistor. Is it ok for me to use series LC instead of only C for parallel Capacitance If I use an inductor to stop RF leaking to ground over capacitance ?

bias.png
 

The impedance seen will be different this way. for a single C impedance @ f=0 is infinity and decays to zero as freq increases but for an LC it is infinity @ f=0 and f=infinity and zero at the resonance freq of the LC. so if you cared about all these differences and they're not gonna change the behavior of your circuit it's ok to use it. bye the way i don't know why you want to do this? what is the benefit of adding an inductor?
 

Benefit of adding inductor is to stop RF going to ground over the Capacitor. I used 50 nH inductor for 7-8 GHz range so it would have impedance of 2280796 ohms for RF signal. So I ensure RF input goes directly to transistor.(the capacitor alone at this point has only 0.22 ohms of impedance so rf would leak over it)
 

Ok, as I understood you want the branch to be high impedance @ RF freq you are working at.This series LC has got a zero, so it will act like a notch filter for the resonance freq. it seems the resonance freq is far below the RF freq which means it is not gonna change the behavior of your circuit.
but just another question, I'm mixed up! use of such a high impedance branch is for biasing, isn't it? and now i consider the thread name is about biasing, so why you are using a cap in series?! the capacitor will block the DC then!
 

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