+ Post New Thread
Results 1 to 9 of 9
  1. #1
    Full Member level 3
    Points: 995, Level: 7

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

    Stability of a circuit and its role in design steps

    Hi,
    This may be a very basic question but I always had my own views on it. When should one check for stability of circuit in the design process, is it before matching the circuit or after?. Usually I check the k-factor before input and output matching, and then make the circuit stable (say by resistive source degenration). After that I perform matching. However, after matching the circuit even if I remove the degenrative resistor, the circuit is still stable. So I ould like to know what is the ideal way of doing it. Thanks

    •   AltAdvertisement

        
       

  2. #2
    Advanced Member level 5
    Points: 29,940, Level: 42
    BigBoss's Avatar
    Join Date
    Nov 2001
    Location
    Turkey
    Posts
    4,372
    Helped
    1318 / 1318
    Points
    29,940
    Level
    42

    Re: Stability of a circuit and its role in design steps

    Overall Stability depends also on "Viewed Impedance" by active device.in fact there are 2 stability metrics..
    -Intrinsic Stability of the active device that is variable by Presented Impedance to the active device ( hint : Stability Circles )
    -Overall Stability of the circuit..( Intrinsic Stability+Extrinsic affects ) .This is more valid for single stage circuits.For more stages , NDF technique can be applied.
    So, the design should following steps for stability..
    *Assure the Intrinsic Stability as much as possible ( if the stability is not maintained, second step can still also be applied )
    *Design Matching Network by aid of first step
    *Check overall stability


    1 members found this post helpful.

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

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

    Re: Stability of a circuit and its role in design steps

    Quote Originally Posted by BigBoss View Post
    Overall Stability depends also on "Viewed Impedance" by active device.
    Yeah the viewed impedance has to be greater than 0.

    Quote Originally Posted by BigBoss View Post
    *Assure the Intrinsic Stability as much as possible ( if the stability is not maintained, second step can still also be applied )
    So it doesn't matter whether the circuit is stable or not, we can still design matching network. I have two questions here
    1. You said "Assure the Intrinsic Stability as much as possible"- How do you do that ? increasing transistor width?

    Quote Originally Posted by BigBoss View Post
    *Check overall stability
    2.What if the circuit is not stable at the end? So far I used degenerative inductor only to stabilize the circuit. Which else techniques do you



    •   AltAdvertisement

        
       

  4. #4
    Advanced Member level 5
    Points: 16,320, Level: 30
    pancho_hideboo's Avatar
    Join Date
    Oct 2006
    Location
    Real Homeless
    Posts
    2,496
    Helped
    664 / 664
    Points
    16,320
    Level
    30

    Re: Stability of a circuit and its role in design steps

    You have to learn followings.

    - Uncoditionally Stable
    - Conditionally Stable

    See https://designers-guide.org/forum/Ya...1218635777/0#5

    Quote Originally Posted by circuitking View Post
    After that I perform matching.
    However, after matching the circuit even if I remove the degenrative resistor,
    the circuit is still stable.
    Conditionally Stable.

    Quote Originally Posted by circuitking View Post
    1. You said "Assure the Intrinsic Stability as much as possible"- How do you do that ?
    increasing transistor width?
    Large width results in more unstable, since large feedback capacitor and large phase delay.

    Quote Originally Posted by circuitking View Post
    2.What if the circuit is not stable at the end?
    So far I used degenerative inductor only to stabilize the circuit.
    It can not be.

    Quote Originally Posted by circuitking View Post
    Which else techniques do you
    Frequency dependent dumping.
    Neutrization.

    Search my very old posts, more than 10 years ago.
    Last edited by pancho_hideboo; 23rd June 2019 at 13:55.



    •   AltAdvertisement

        
       

  5. #5
    Full Member level 3
    Points: 1,131, Level: 7

    Join Date
    Jan 2019
    Location
    Belgium
    Posts
    163
    Helped
    66 / 66
    Points
    1,131
    Level
    7

    Re: Stability of a circuit and its role in design steps

    I see it as well. The losses in the matching network cause the amplifier to be stable. This is what I did for single ended amplifiers:
    1. Design the amplifier with a series gate resistance/source degeneration such that the amplifier is stable. I try to keep Kf close to 1 and not overdo it
    2. Design the output and input matching networks, and make sure to include losses in them.
    3. Check the stability. You will now see that the amplifier has a much larger Kf and the gain is reduced.
    4. I remove/reduce the degeneration resistance and then tune the parameters of the input matching network so that s11 reduces.
    5. I disconnect the output matching network, re-run the loadpull and then design the output matching network.
    And then I keep iterating between steps 4 and 5.

    But for mmwave, the differential topology is better as I can use capacitive neutralization.



  6. #6
    Full Member level 3
    Points: 995, Level: 7

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

    Re: Stability of a circuit and its role in design steps

    Quote Originally Posted by BigBoss View Post
    ( if the stability is not maintained, second step can still also be applied )
    Actually I realized that to do Harmonic balance simulation the circuit should be stable but for this I need to extra component to make it stable. But how to do I find the exact Output power of a gain stage if I connect the extra components .

    - - - Updated - - -

    Quote Originally Posted by vivekroy View Post
    1. Design the amplifier with a series gate resistance/source degeneration such that the amplifier is stable. I try to keep Kf close to 1 and not overdo it
    I also kept series gate resistance to make it stable but how is going to impact the output power I get from the gain stage. Because I am only interested in the power offered by the gain stage without the resistor



  7. #7
    Full Member level 3
    Points: 1,131, Level: 7

    Join Date
    Jan 2019
    Location
    Belgium
    Posts
    163
    Helped
    66 / 66
    Points
    1,131
    Level
    7

    Re: Stability of a circuit and its role in design steps

    Quote Originally Posted by circuitking View Post
    Actually I realized that to do Harmonic balance simulation the circuit should be stable but for this I need to extra component to make it stable. But how to do I find the exact Output power of a gain stage if I connect the extra components .

    - - - Updated - - -



    I also kept series gate resistance to make it stable but how is going to impact the output power I get from the gain stage. Because I am only interested in the power offered by the gain stage without the resistor
    You only place the gate resistor to make it stable (Kf=1) which means your MAG and MSG are equal. You cannot get a gain more than MSG anyway. This is just to aid in the first iteration of doing a loadpull. Later on when you start introducing losses in your matching network, you can get rid of your series gate resistance. That gain will be typically lesser than the gain you get when you use the series gate resistance to get a Kf=1.


    1 members found this post helpful.

    •   AltAdvertisement

        
       

  8. #8
    Advanced Member level 5
    Points: 16,320, Level: 30
    pancho_hideboo's Avatar
    Join Date
    Oct 2006
    Location
    Real Homeless
    Posts
    2,496
    Helped
    664 / 664
    Points
    16,320
    Level
    30

    Re: Stability of a circuit and its role in design steps

    Quote Originally Posted by circuitslave View Post
    Actually I realized that to do Harmonic balance simulation the circuit should be stable
    No.
    Do pure frequency domain HB Analysis without transient assist, it can converge even if your circuit is unstable.

    Probably you can not understand anything at all.

    Learn and understand HB Analysis surely.
    https://www.edaboard.com/showthread.php?385490#4

    Even if HB Analysis can converge, your circuit gives oscillation in real world.
    So your circuit is not useful.
    Last edited by pancho_hideboo; 7th August 2019 at 14:17.


    1 members found this post helpful.

  9. #9
    Full Member level 3
    Points: 1,131, Level: 7

    Join Date
    Jan 2019
    Location
    Belgium
    Posts
    163
    Helped
    66 / 66
    Points
    1,131
    Level
    7

    Re: Stability of a circuit and its role in design steps

    Quote Originally Posted by pancho_hideboo View Post
    No.
    Do pure frequency domain HB Analysis without transient assist, it can converge even if your circuit is unstable.

    Probably you can not understand anything at all.

    Learn and understand HB Analysis surely.
    https://www.edaboard.com/showthread.php?385490#4

    Even if HB Analysis can converge, your circuit gives oscillation in real world.
    So your circuit is not useful.
    I know its not relevant to this thread particularly, but it is particularly relevant to HB not converging for conditionally stable circuits.

    I was once designing a PA using bipolar devices at mmwave frequencies. When I used the Gummel Poon model (which does not capture a lot of high current effects which are very relevant to the operation of a PA) using pure frequency domain HB analysis, even if the circuit was not unconditionally stable, the simulation converged. But when I switched to HiCUM model (which is a very detailed model that includes a lot of high current effects), if it was not unconditionally stable, it would fail and mostly for high power points. So I don't think its just pure stability. The model files used also have a role to play.



--[[ ]]--