Automatic voltage switching circuits for providing a higher voltage of multiple supply voltages are disclosed. In one aspect, an automatic voltage switching circuit is configured to generate a compare signal indicating which of a first supply voltage and a second supply voltage is a higher voltage. The automatic voltage switching circuit is further configured to generate first and second select signals based on the compare signal, wherein the first and second select signals are in a voltage domain of the higher voltage. Transistors corresponding to the first and second supply voltages control switching the output voltage to the higher voltage in response to the first and second select signals. Biasing the back-gates of the transistors using the output voltage reduces or avoids forward biasing in the body diodes of the transistors, thus reducing the possibility of the output voltage causing interference on a power supply corresponding to a non-activated transistor.