Provided herein are circuits and methods to generate a voltage proportional to absolute temperature (VPTAT) and/or a bandgap voltage output (VGO). A circuit includes a group of X transistors. A first subgroup of the X transistors are used to produce a first base-emitter voltage (VBE1). A second subgroup of the X transistors are used to produce a second base-emitter voltage (VBE2). The VPTAT can be produced by determining a difference between VBE1 and VBE2. Which of the X transistors are in the first subgroup and used to produce the first base-emitter voltage (VBE1), and/or which of the X transistors are in the second subgroup and used to produce the second base-emitter voltage (VBE2), change over time. Additionally, a circuit portion can be used to generates a voltage complimentary to absolute temperature (VCTAT) using at least one of the X transistors. The VPTAT and the VCTAT can be added to produce the VGO.