Mustafa Ahmad10/19/2021, 8:47 AM
Luis Henrique Rodovalho10/19/2021, 11:09 AM
Stefan Schippers10/20/2021, 10:27 AM
, no current flowing in the resistors and in the bipolar transistors, all nodes are at a low voltage (may be 0.3 / 0.4V), depending on the Differential amplifier offset circuit may remain stuck in this state. The 'MX' MOS put in parallel with the left differential input MOS forces the DRIVE node low when Vbgap = V+ = V-, so the Vbgap node will be pulled high. After the bandgap settles to its correct operating point Vbgap=~1.25, while V+ = V- = ~0.65V, so the MX transistor will be in the OFF state not disturbing the differential amplifier. If for some reason the bandgap switches off again the MX will again pull the Vbgap high. Note that this is a very old design on a 5V technology. For 1.8V only different solutions must be adopted. See the
V+ = V- = Vbgap
example circuit provided in the xschem_sky130, that has the same startup mechanism and also includes dynamic opamp offset cancellation.