A current source circuit includes a first transistor Q1, a second transistor Q2, a first resistor R1, and a second resistor R2. The first transistor Q1 has a first terminal (collector) coupled with the supply voltage (VCC), a second terminal (base) coupled with the first resistor R1, and a third terminal (emitter) coupled with the second resistor R2. The second transistor Q2 has a first terminal coupled with the second terminal of the first transistor Q1, a second terminal coupled with the third terminal of the first transistor Q1, and a third terminal coupled with the filtering circuit 39. The first resistor R1 is coupled between the supply voltage and a second terminal of the first transistor Q1. The second resistor R2 is coupled between a third terminal of the first transistor Q1 and the filtering circuit 39.