Practical Current Source

 Practical Current Source

Practical Current Source :

 

An Ideal current source is simply a thought. A perfect current source can't be existed, because there's no source which will supply constant current albeit its terminals are open-circuited.

 

A practical current source doesn't work like a perfect current source because a practical source has an internal  impedance which isn't infinite. A practical current source is represented by the symbol shown in figure below:

The source impedance Z_S is put in parallel with the perfect current source I_S. Now, if we connect a load across the terminals A and B, the load current are going to be different from the present I_S. The present I_S now divides itself between two branches

 

1. One made from source impedance Z_S inside the source itself,

2. And the other made from the load impedance Z_L external to the source.

Let us the conditions under which a source can work as an honest (practical) current source.

 

In the following figure features a load impedance Z_L is connected to a current source. :

 

Let I_S be the short-circuit current of the source, and Z_S be its internal impedance. the present I_S is seen to be divided into parts

 

IS through Z_S

And IL through Z_L.

 

That is , I_S = I₁ + I_L

 

Or I₁ = I_S - I_L

 

Since the impedance Z_S and Z_L are in parallel, the drop across each should be equal, I.e.,

 

 I₁ Z_S = I_L Z_L

 

Or (I_S - Z_L) Z_S = I_L Z_L

 

Or I_L = I_S Z_S / Z_S + Z_L

 

Or I_L = I_S / 1+(Z_L / Z_S)

 

This equation tells us that the load current I_L will remain almost an equivalent because the current I_S , provided the ratio Z_L / Z_S is little compared to unity. The source then behaves as an honest current source.

 

In other words, the larger the worth of internal impedance Z_S (compared to the load impedance ZL), The smaller is that the ratio Z_L / Z_S, and therefore the better it works as a continuing current source.

From equation

 

 I_L = I_S / 1+(Z_L / Z_S)

 

We see that the present I_L = I_S, when Z_L = 0. But, because the value of load impedance is increased, the present I_L is reduced. For a given increase in load impedance Z_L, the corresponding reduction in load current I_L is far smaller. Thus with the rise in load impedance, the interior terminal voltage (V = I_L Z_L) also increases

 

The V-I characteristic of a practical current source is shown below: