r 11 230 and r 12 232 of the second circuit 204 sense the voltage from the second voltage source 256 the sensed voltage of the first voltage source 254 is provided to the non inverting pin 212 of the first comparator 210 and the inverting pin 236 of the second comparator 234 the sensed voltage of the second voltage source 256 is provided to the inverting pin 214 of the first comparator 210 and the non inverting pin 238 of the second comparator 234 assume that a first voltage from the first voltage source 254 is approximately u2212 48v and a second voltage from the second voltage source 256 is approximately 0v i e the first voltage source 254 is u201c on u201d and the second voltage source 256 is u201c off u201d at the first comparator 210 the voltage at the non inverting pin 212 is greater than the voltage at the inverting pin 214 the voltage at the output pin 216 is thus approximately u2212 vcc a voltage of u2212 vcc is applied to the gate of q 3 218 this turns q 3 218 u201c on u201d turning q 3 218 u201c on u201d also turns q 1 226 u201c on u201d a path is thus created for a current in the first circuit 202 such that approximately u2212 48v is provided at the output 258 by the first voltage source 254 at the second comparator 234 the voltage at the inverting pin 236 is greater than the voltage at the non inverting pin 238 the voltage at the output pin 240 is thus approximately.