# what happens to RF choke while doing ac analysis

1. ## what happens to RF choke while doing ac analysis

Hi, I know how to find dc analysis (to find gm) and ac analysis (to find gain) for a given circuit (Say CS stage) when resistive load (say R)is there, i.e. Resistor is connected between Vdd and drain terminal. From this I can arrive at the conclusion that voltage gain Av=-gm*(rds//R).

But what happens if I have a RF choke conected between Vdd and drain terminal. While performing AC analysis inductor (RF choke) should become open circuit which leaves drain terminal to be open and then the calculated voltage gain Av=-gm*rds. Is this correct or should we take the impedance of RF choke and include it also in the calculations?.

I also heard from a friend that there is something called RF anlysis also just like DC and AC analysis. I am wondering what that might be, anyone has any idea about it

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2. ## Re: what happens to RF choke while doing ac analysis

RF Choke will act as Open Circuit for AC Signals and Av=-gm*rds will be as you have remarked ( Open Circuit Voltage Gain ). Another advantage of using RFC is that'll give you more Voltage Headroom for Vds ( no voltage drop on it ).These are however valid for ;

*Reactance of the RFC is sufficiently high for lowest AC frequency
*Self Resonance has never been taken into account ( it ought to be in practice )

AC Signal Analysis is general purpose and it is used very rarely in RF designs. But s-parameters analysis and especially HB analysis are used essentially in RF Circuits.

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3. ## Re: what happens to RF choke while doing ac analysis

Originally Posted by BigBoss
*Self Resonance has never been taken into account ( it ought to be in practice )
Could you explain more about this ? does this mean at the self-resonance frequency the RFC will have an internal resistor and I should take that into consideration?

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Originally Posted by BigBoss

AC Signal Analysis is general purpose and it is used very rarely in RF designs. But s-parameters analysis and especially HB analysis are used essentially in RF Circuits.
Is there a recommanded procedure to perform S-parameter analysis and HB analysis, just like AC and DC analysis, to understand easily for beginners? Any good book or tutorial?

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4. ## Re: what happens to RF choke while doing ac analysis

-Self Resonance Frequency is the functional limit of the RFC so that RFC cannot be used beyond SFR due to changing behavior of its nature.( beyond SFR, the RFC is not Inductor anymore, instead it's a dominantly capacitor )
-For s-parameters analysis and HB analysis, see the Help of the simulator which you will choose and search internet because there are lots of docs.

5. ## Re: what happens to RF choke while doing ac analysis

do you mean "RF CHOKE", which is an inductor that deliberately has high loss, and an "INDUCTOR", which deliberately has low loss (i.e. high Q)? Makes a big difference in the answer

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6. ## Re: what happens to RF choke while doing ac analysis

Originally Posted by biff44
do you mean "RF CHOKE", which is an inductor that deliberately has high loss, and an "INDUCTOR", which deliberately has low loss (i.e. high Q)? Makes a big difference in the answer
I believe it means high impedance at high (relevant) frequencies. Whether by inductance or not.

Usually preferrable by inductance, since DC bias gets affected if it's due to resistance.

Having said that, I do not think an element designated as an RF choke needs to be designed with hi-Q. That's more relevant for tuned circuits and such applications.

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