The present invention provides a long time constant integrator circuit as part of an integrated circuit. The integrator circuit is fully integrated on chip with no external capacitive or resistive components for enhancing the circuit's time constant. It achieves a −3 dB cut-off frequency of 1.6 Hz. The circuit is realisable on a very small area of silicon being formed by a bipolar process using npn transistors, resistive and capacitive elements. The integrator circuit comprises a transconductance stage as an input to an operational amplifier. The circuit design is fully differential and employs realisable resistors and capacitors.