+ Post New Thread
Results 1 to 6 of 6
  1. #1
    Full Member level 3
    Points: 995, Level: 7

    Join Date
    Jan 2018
    Posts
    157
    Helped
    1 / 1
    Points
    995
    Level
    7

    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).

    Click image for larger version. 

Name:	IMG_20190709_193700.jpg 
Views:	5 
Size:	467.3 KB 
ID:	154326

    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

    •   AltAdvertisement

        
       

  2. #2
    Advanced Member level 5
    Points: 29,961, Level: 42
    BigBoss's Avatar
    Join Date
    Nov 2001
    Location
    Turkey
    Posts
    4,376
    Helped
    1318 / 1318
    Points
    29,961
    Level
    42

    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.



    •   AltAdvertisement

        
       

  3. #3
    Full Member level 3
    Points: 995, Level: 7

    Join Date
    Jan 2018
    Posts
    157
    Helped
    1 / 1
    Points
    995
    Level
    7

    Re: what happens to RF choke while doing ac analysis

    Quote Originally Posted by BigBoss View Post
    *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?

    - - - Updated - - -

    Quote Originally Posted by BigBoss View Post

    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?



    •   AltAdvertisement

        
       

  4. #4
    Advanced Member level 5
    Points: 29,961, Level: 42
    BigBoss's Avatar
    Join Date
    Nov 2001
    Location
    Turkey
    Posts
    4,376
    Helped
    1318 / 1318
    Points
    29,961
    Level
    42

    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. #5
    Advanced Member level 5
    Points: 34,278, Level: 45

    Join Date
    Dec 2004
    Location
    New England, USA
    Posts
    4,814
    Helped
    1347 / 1347
    Points
    34,278
    Level
    45

    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



    •   AltAdvertisement

        
       

  6. #6
    Advanced Member level 4
    Points: 6,247, Level: 18
    kripacharya's Avatar
    Join Date
    Dec 2012
    Location
    New Delhi
    Posts
    1,209
    Helped
    182 / 182
    Points
    6,247
    Level
    18

    Re: what happens to RF choke while doing ac analysis

    Quote Originally Posted by biff44 View Post
    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.



--[[ ]]--