in the temperature or power supply the ptat circuit 204 comprises two pnp bipolar transistors 210 and 212 a resistor 214 two pmos transistors 216 and 218 and a differential amplifier 220 the ctat circuit 202 comprises a pnp bipolar transistor 222 a resistor 224 two pmos transistors 226 and 228 and a differential amplifier 230 the ptat circuit 204 is designed to operate in a manner similar to the conventional bandgap reference circuit 102 in which two stable operating points are provided to allow a simple start up circuit such as the start up circuit 104 to be used to reliably start up the ptat circuit 204 and to activate the bandgap reference circuit 200 the start up circuit 104 comprised of the nmos transistor 128 and the two pmos transistors 130 and 132 is connected to the ptat circuit 204 at a node 232 through the gate of the pmos transistor 130 and at a node 234 through the drain of the pmos transistor 132 the sources of the pmos transistors 130 and 132 and the gate of the nmos transistor 128 are all tied to the voltage source while the drain of the nmos transistor 128 the gate of the pmos transistor 132 and the drain of the pmos transistor 130 are all coupled together at the node 136 when the supply voltage is applied at the beginning of operation the nmos transistor 128 is turned on thus pulling the node 136 low to ground this turns on the pmos transistor 132 thus pulling the node 234 high to the supply voltage with the help of the start up circuit 104 the current of.