a current mirror includes a serially connected diode - connected transistor of a first conductivity type , a saturated transistor of a second conductivity type , and a current source for providing a reference current . a gate voltage generated by the diode - connected transistor in response to the reference current is provided to the gate of a matching transistor . this causes the matching transistor to mirror the reference current . meanwhile , an output transistor cascoded with the matching transistor is gate - coupled to the junction between the saturated transistor and the current source . this allows the output transistor to provide an output voltage swing from one supply voltage to two saturation voltage drops from the second supply voltage . meanwhile , the cascode configuration gives the current mirror a high output impedance .