An apparatus, comprising: a calibration circuit configured to generate a reference signal (V REF); and at least one differential circuit (110, 120) each being configured to operate at a calibrated transconductance over process or condition variations based on the reference signal (V REF), wherein the calibration circuit is further configured to generate the reference signal (V REF) independent of the at least one differential circuit (110, 120) and to provide a feedback of the reference signal (V REF) and adjust the reference signal (V REF) based on the feedback, wherein the calibration circuit further comprises an operational transconductance amplifier, OTA, configured to generate a tail current (I TAIL) based on the feedback and to keep the reference signal (V REF) constant over process or condition variations; wherein the calibration circuit further comprises a circuit configured to convert a reference current (Ip) into a reference voltage (V IN), and wherein the OTA is further configured to generate an output based on the reference voltage (V IN); wherein the calibration circuit is further configured to keep the reference voltage (V IN) constant over the process or condition variations; wherein the calibration circuit further comprises an operational amplifier configured to compare a voltage (V 1) based on the output of the OTA and a second reference voltage (V 0), wherein the calibration circuit is configured to generate the reference signal (V REF) based on the comparison; wherein the second reference voltage (V 0) is based on a second reference current (I 0), and wherein the second reference current (I 0) is constant over the process or condition variations. a calibration circuit configured to generate a reference signal (V REF); and at least one differential circuit (110, 120) each being configured to operate at a calibrated transconductance over process or condition variations based on the reference signal (V REF), wherein the calibration circuit is further configured to generate the reference signal (V REF) independent of the at least one differential circuit (110, 120) and to provide a feedback of the reference signal (V REF) and adjust the reference signal (V REF) based on the feedback, wherein the calibration circuit further comprises an operational transconductance amplifier, OTA, configured to generate a tail current (I TAIL) based on the feedback and to keep the reference signal (V REF) constant over process or condition variations; wherein the calibration circuit further comprises a circuit configured to convert a reference current (Ip) into a reference voltage (V IN), and wherein the OTA is further configured to generate an output based on the reference voltage (V IN); wherein the calibration circuit is further configured to keep the reference voltage (V IN) constant over the process or condition variations; wherein the calibration circuit further comprises an operational amplifier configured to compare a voltage (V 1) based on the output of the OTA and a second reference voltage (V 0), wherein the calibration circuit is configured to generate the reference signal (V REF) based on the comparison; wherein the second reference voltage (V 0) is based on a second reference current (I 0), and wherein the second reference current (I 0) is constant over the process or condition variations.