Here, we have presented several CM structures, summary of which is presented in Table 1.1. One can select an appropriate CM for a particular application. For example in low -voltage application, simple CM, wide -swing CM and enhanced output impedance CM can be selected because they require low compliance voltages at the output node. However, the output impedance of the simple CM is too low, and it may not be possible to use these mirrors in most of the application. Thus, the choice falls on using wide -swing and enhanced output impedance CM. Since the structure of enhanced output impedance CM is complicated, generally their use is restricted to special type of applications, where their use cannot be avoided. Often we require tight matching between input and output currents. This in turn requires tight matching between the device dimensions. This problem of device mismatches, however, has not been addressed. Further the current obtainable from the CMs should be invariant to the supply voltages and/or temperatures changes between some specified limits, which have not been discussed here.