+ Post New Thread
Results 1 to 5 of 5
  1. #1
    Member level 2
    Points: 514, Level: 4

    Join Date
    Jan 2018
    Posts
    45
    Helped
    1 / 1
    Points
    514
    Level
    4

    Pulse distortion after amplification

    Hi guys!

    I've designed a simple common emitter LNA at 10GHz, i ve used it in many CW applications with no major problems. However i tried to amplify a 4us pulse (a cosine of constant magnitude) and i notice a pulse distortion when the LNA is saturated, where the signal magnitude is higher at the begining of the pulse and it keeps dropping for 500~1000ns, until it reaches a constant magnitude for the rest of the pulse, i tried filtering the harmonics and while this helps a little bit the distortion is still present. have any way of you encountred the same problem? or have an idea wht it might be?

    Thank you in advance. I can provide the schematic/Layout of the LNA if required.

    •   AltAdvertisement

        
       

  2. #2
    Super Moderator
    Points: 260,190, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    45,437
    Helped
    13823 / 13823
    Points
    260,190
    Level
    100

    Re: Pulse distortion after amplification

    I can provide the schematic/Layout of the LNA if required.
    Good idea. I guess you have input or output AC coupling and the bias point is shifting when the input signal is rectified in saturated operation.



    •   AltAdvertisement

        
       

  3. #3
    Advanced Member level 5
    Points: 33,915, Level: 44

    Join Date
    Oct 2001
    Posts
    4,841
    Helped
    1436 / 1436
    Points
    33,915
    Level
    44

    Re: Pulse distortion after amplification

    By definition an LNA should NOT be saturated in any cases.
    Reduce the input level into LNA, or use a higher IP3 device (with higher P1dB) which can handle higher input levels.



    •   AltAdvertisement

        
       

  4. #4
    Member level 2
    Points: 514, Level: 4

    Join Date
    Jan 2018
    Posts
    45
    Helped
    1 / 1
    Points
    514
    Level
    4

    Re: Pulse distortion after amplification

    Here is the layout of the
    Click image for larger version. 

Name:	Capture1.PNG 
Views:	2 
Size:	56.6 KB 
ID:	155602

    and this is the bias circuitry
    Click image for larger version. 

Name:	2.PNG 
Views:	4 
Size:	33.4 KB 
ID:	155603

    By definition an LNA should NOT be saturated in any cases.
    Reduce the input level into LNA, or use a higher IP3 device (with higher P1dB) which can handle higher input levels
    The application where i m using it won't require the LNA to be saturated all the time, but at the beginning, it's going to be saturated for a small laps of time (i need to aquire that pulse too) and then it'll go back to being linear. i have tested other amplifiers (from mini-circuit) and they dont have that probleme even when saturated, so i know it s my design that sucks.



  5. #5
    Advanced Member level 5
    Points: 34,356, Level: 45

    Join Date
    Dec 2004
    Location
    New England, USA
    Posts
    4,820
    Helped
    1349 / 1349
    Points
    34,356
    Level
    45

    Re: Pulse distortion after amplification

    Quote Originally Posted by FvM View Post
    Good idea. I guess you have input or output AC coupling and the bias point is shifting when the input signal is rectified in saturated operation.
    i agree, the bias point is shifting. Possibly from the input of your active device rectifying the signal?
    You need to bias it up from a "stiffer" source.

    IF it were a large signal amplifier, you might have to also consider the device heating up during the pulse, and shifting operating point...but you said an LNA...


    1 members found this post helpful.

--[[ ]]--