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.

ljp2706 said:

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.

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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).