A circuit includes an amplifier having first and second inputs and an output, and a feedback circuit configured to generate a feedback voltage in response to a voltage at the output of the amplifier. The feedback circuit is coupled to the first input of the amplifier to provide the feedback voltage to the first input of the amplifier. An output circuit is configured to generate a variable bias current in response to the voltage at the output of the amplifier. A switch circuit is configured to switch the second input of the amplifier from receiving a first reference voltage during a first mode of operation to receiving a second reference voltage during a second mode of operation.