Practical Current source

Practically the current sources don’t have infinite resistance across them but has 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. The characteristics of the practical current source can be obtained as shown in the below figure. 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 below circuit show the practical current source in which the current delivered from the source is indicated by I_s and its internal resistance with a resistor R. I_t is the current delivered to the external circuit.

Practical current source

The current to delivered to the external circuit I_t is given by,
I_t=I_s-(V/R)

V-I characteristics

The below figure shows the V-I characteristics of a practical current source along with its comparison with the characteristic of ideal current source. The reduction of the current is due the internal resistance R.

Characteristics of a practical current source