V-I  Characteristics  of  Constant  Current  Source

A  source  which  has  high  internal resistance  (or  impedance)  as  compared  to  load  resistance  (or  impedance)  is considered as a constant current source.

A real current source has high internal resistance. For example a real current source having an internal resistance of 10 M? with load resistance R_L is illustrated in below fig. (c). When load resistance varies from 1 K? to 100 K?,

∴Current varies from 1.19988 µA to 1.1881 µA.

This illustrates that load current remains almost constant and source behaves as a constant  current  source  irrespective  of  value  of  load  resistance,  which  is  illustrated in fig.(d).