A circuit includes an operational amplifier including a first input and a second input. A first resistor has a first end coupled to the first input. A first bipolar transistor includes a first emitter coupled to a second end of the first resistor, and a first base. A second bipolar transistor includes a second emitter coupled to the second input, and a second base. A third bipolar transistor includes a third emitter coupled to the first base, a first collector, and a third base connected to the first collector. A fourth bipolar transistor includes a fourth emitter coupled to the second base, a second collector, and a fourth base connected to the second collector. A second resistor is coupled to the first input, wherein the second resistor is parallel to the first resistor and the first bipolar transistor.