Second, the mirror requires a small voltage drop to maintain operation (to keep the output transistors of the mirror in active mode). As a result, the current mirror does limit the allowable output voltage swing, but this limitation is much less than for a resistor, and also does not depend upon the choice of bias current, leaving more flexibility than a resistor in designing the circuit.
The mirror requires a small voltage drop for operation (to maintain the output transistors in active mode), limiting the allowable output voltage swing, but this limitation is less severe than that of a resistor, and it does not depend on the bias current selection, offering greater design flexibility than a resistor.