Threshold voltage sensor and a chip using the threshold voltage sensor

A threshold voltage sensor, comprising:

• a first metal-oxide-semiconductor field-effect transistor (MOSFET), a second MOSFET, and an operational amplifier, wherein the operational amplifier has an output terminal coupled to a gate terminal of the first MOSFET and a gate terminal of the second MOSFET, a first input terminal receiving a first constant voltage, and a second input terminal coupled to a drain terminal of the first MOSFET;
• an active device, coupled between the drain terminal of the first MOSFET and a ground terminal to determine a constant current that is mirrored from the first MOSFET to the second MOSFET, wherein the active device comprises a third MOSFET coupled between the drain terminal of the first MOSFET and the ground terminal, and the third MOSFET has a gate terminal biased by a second constant voltage;
• a diode-connected MOSFET, coupled between a drain terminal of the second MOSFET and the ground terminal;
• and
• a fourth MOSFET, coupled in series with the diode-connected MOSFET, and having a gate terminal biased by a third constant voltage;
• wherein the constant current flows through the diode-connected MOSFET to generate an output voltage that contains information about a threshold voltage of the diode-connected MOSFET.