Concept of Current Source

 Like a constant voltage source, there may be a constant current source – a source that supplies a constant current to a load even its impedance varies. Ideally, the current supplied by it should remain constant, no matter what the load impedance is.

A symbolic representation of such an ideal current source is shown in Fig. 1(a). The arrow inside the circle indicates the direction in which current will flow in the circuit when a load is connected to the source. Fig. 1(b) shows the V-I characteristic of an ideal current source. Let us connect a variable load impedance ZL to a constant current source as shown in Fig. 1(c). As stated above, the current supplied by the source should remain constant at is for all values of load impedance.

Fig.1: (a) Symbol for an ideal current source (b) V-I characteristic of an ideal current source (c) A variable load connected to an ideal current source (d) Symbol for a practical current source

It means even if ZL is made infinity, the current through this should remain IS (same). Now, we must see if any practical current source could satisfy this condition. The load impedance ZL = ∞ means no conducting path, external to the source, exists between the terminals A and B. Hence, it is a physical impossibility for current to flow between terminals A and B. If the source could maintain a current Is through an infinitely large load impedance, there would have been an infinitely large voltage drop across the load. It would then have consumed infinite power from the source. Of course, no practical source could ever supply infinite power.

The maximum voltage that the current source can deliver to the load is called compliance voltage. During the variation in the load the current source work like ideal source, provides the unlimited resistance but, when the voltage value at the output reaches to compliance voltage, then it starts to behave like a real source and provides the limited value of resistance.

A practical current source supplies current IS to a short-circuit (i.e. when ZL= 0). That is why the current IS is called short-circuit current. But, when we increase the load impedance, the current falls below IS. When the load impedance ZL is made infinite (i.e., the terminals A and B are open-circuited), the load current reduces to zero. It means there should be some path (inside the source itself) through which the current IS can flow. When some finite load impedance is connected, only a part of this current IS flows through the load. The remaining current goes through the path inside the source. This inside path has an impedance ZS, and is called the internal impedance. The symbolic representation of such a practical current source is shown in Fig. 1(d).

Now, if terminals AB are open-circuited (ZL = ∞) in Fig. 1(d), the terminal voltage does not have to be infinite. It is now a finite value, VT = IS ZS. It means that the source does not have to supply infinite power!