An electronic circuit includes a first transistor, a second transistor, and a variable resistor. The first transistor has a first threshold voltage. The second transistor has a second threshold voltage that is different from the first threshold voltage. The second transistor is coupled to the first transistor. The variable resistor is coupled to the first transistor and the second transistor. The variable resistor is configured to adjust a temperature coefficient of the electronic circuit. The electronic circuit is configured to generate a reference voltage based on a difference of the first threshold voltage and the second threshold voltage.