An operational amplifier comprises a first input pair, a second input pair, a switch and a first current mirror. The first input pair comprises a different type of MOS transistor from the second input pair. The switch determines which one of the first or the second input pair is functioning and the operating input pair is configured to output voltage. The switch is further connected to the first input pair and the first current mirror. The first current mirror is further connected to the second input pair, and is configured to copy a current passing through the switch to the second input pair. Therefore an increase of transconductance of the first input pair is compensated by a decrease of transconductance of the second input pair, and the operational amplifier has a substantially constant transconductance no matter which of the first input pair and the second input pair is functioning.