Bias current generation circuit

A circuit, comprising:

• an operation amplifier comprising at least two input terminals and an output terminal, wherein the at least two input terminals are respectively configured to receive an input voltage having a zero-temperature coefficient and a feedback voltage to generate a driving voltage at the output terminal according to a comparison result between the input voltage and the feedback voltage;
• an output transistor configured to generate a bias current according to the driving voltage;
• and
• a variable resistive circuit electrically coupled to the output transistor through a feedback node and configured to generate the feedback voltage according to the bias current, wherein the variable resistive circuit comprises:
• a plurality of resistors electrically coupled in series each having a load resistance and a positive-temperature coefficient and each having a current input terminal and a current output terminal;
• and
• a plurality of switch transistors each electrically coupled between the current output terminal of one of the resistors and a ground terminal, wherein one of the switch transistors turns on according to a control voltage variable with a temperature change to enable the corresponding one of the resistors and generates a transistor resistance having a negative temperature coefficient.