A circuit comprises a first amplifier and a second amplifier. The first amplifier is configured to amplify a first voltage difference between a first voltage and a second voltage, and to generate a third voltage. The second amplifier is configured to amplify a second voltage difference between the third voltage and an input voltage, and to generate an output voltage. The first voltage is a voltage at a first terminal of a first transistor. The second voltage is a voltage at a second terminal of a second transistor. A first gate of the first transistor is adapted to receive the third voltage. A second gate of the second transistor is adapted to receive the input voltage. Threshold voltage values of the first transistor and the second transistor differ.