Practically, the current sources don’t have infinite resistance across them but have some finite and high resistance. Due to the finite resistance, the practical current source shows some dependency on the voltage across it. So the current delivered by a practical current is not a constant as in case of the ideal one.
A practical current source is modelled as an ideal current source in parallel with its internal resistance indicated by a resistor.
Circuit of a practical current source
The Fig. 1 shows the practical current source in which the current delivered from the source is indicated by Is and its internal resistance with a resistor Rs. It is the current delivered to the external circuit.
The current to delivered to the external circuit It is given by,
The Fig. 2 shows the V-I characteristics of a practical current source along with its comparison with the characteristic of an ideal current source. The reduction of the current is due to the internal resistance Rs.