+ Post New Thread
Results 1 to 2 of 2
  1. #1
    Full Member level 1
    Points: 1,344, Level: 8

    Join Date
    Mar 2015
    Location
    USA
    Posts
    109
    Helped
    20 / 20
    Points
    1,344
    Level
    8

    Brokaw Cell Start Up Question

    In a Brokaw cell during startup, how high does vbg need to be pulled up by the startup mechanism to snap out of the undesired operating point? I have heard it may be more than a VBE.

    •   AltAdvertisement

        
       

  2. #2
    Advanced Member level 5
    Points: 39,152, Level: 48

    Join Date
    Mar 2008
    Location
    USA
    Posts
    6,329
    Helped
    1843 / 1843
    Points
    39,152
    Level
    48

    Re: Brokaw Cell Start Up Question

    Quote Originally Posted by ljp2706 View Post
    In a Brokaw cell during startup, how high does vbg need to be pulled up by the startup mechanism to snap out of the undesired operating point? I have heard it may be more than a VBE.
    If it's an all-bipolar PTAT then you just need about a Vbe,
    enough that the current ratio is something like the explicit
    area ratio. Then the loop should take over. These PTATs
    tend to use a diode-steered current from a boot JFET or
    high value resistor, and getting the ratios / currents to stay
    right over PVT is a challenge especially once you throw in
    pessimistic mismatch statistics.

    CMOS / BiCMOS hybrids, like the ones that use an entire
    CMOS op amp to stuff current down the ratioed pair, may
    need a totally different boot-method (like a common source
    switch whose gate looks at Vbg, drain drives another switch's
    gate whose drain pulls on the high side mirror rack in parallel
    with the op amp (such that Vbg < VT(N) causes boot current
    to be sunk).



--[[ ]]--