+ Post New Thread
Results 1 to 4 of 4
  1. #1
    Full Member level 5
    Points: 3,162, Level: 13
    Achievements:
    7 years registered

    Join Date
    May 2010
    Location
    Poland
    Posts
    248
    Helped
    5 / 5
    Points
    3,162
    Level
    13

    Rise times and fall times of fast transistors seem way too long

    In the data sheet of a 400 MHz transistor I found fall time = 60 ns. Clearly these are contradictory specifications
    - trise and tfall should be at least an order of magnitude shorter.
    Should I believe that fT is specified correctly? (e.g. 2SA2088FRAT106Q transistor).

    •   AltAdvertisement

        
       

  2. #2
    Super Moderator
    Points: 82,095, Level: 69
    Achievements:
    7 years registered
    Awards:
    Most Frequent Poster 3rd Helpful Member

    Join Date
    Apr 2014
    Posts
    16,663
    Helped
    3778 / 3778
    Points
    82,095
    Level
    69

    Re: Rise times and fall times of fast transistors seem way too long

    Hi,

    400MHz is for small signal (non saturated) analog mode....where the gain = 1
    60ns is for large signal, saturated, switching mode.

    Two completely different situations.

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.


    1 members found this post helpful.

    •   AltAdvertisement

        
       

  3. #3
    Super Moderator
    Points: 265,442, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    46,381
    Helped
    14116 / 14116
    Points
    265,442
    Level
    100

    Re: Rise times and fall times of fast transistors seem way too long

    Apart from the difference between saturated switching and small signal amplifier operation, do you understand the meaning of fT? It's the frequency where the small signal current gain would drop to unity.



    •   AltAdvertisement

        
       

  4. #4
    Advanced Member level 5
    Points: 41,039, Level: 49

    Join Date
    Mar 2008
    Location
    USA
    Posts
    6,610
    Helped
    1951 / 1951
    Points
    41,039
    Level
    49

    Re: Rise times and fall times of fast transistors seem way too long

    fT of an unloaded transistor, or one in a 50 ohm ideal
    test setup and rise/fall times with a spec load, 10-90%
    are not at all the same condition.

    Fall time is drive current and (load+Miller) capacitance.



--[[ ]]--