current mirror circuit using bjt

✨current mirror circuit using BJT 

The current mirror is a dc network in which the current through a load(Iout) is controlled by a current(Iref) at another point in the network. That is if the controlling current is raised or lowered the current through the load will change to the same level. The discussion to follow will demonstrate that the effectiveness of the design is dependent on the fact that the two transistors employed have identical characteristics. The basic configuration appears in Fig. Note that the two transistors are back to back and the collector of one is connected to the base of the two transistors.

 Implementation of basic current mirror circuit :

the above diagram shows the basic current mirror circuit, now to understand the concept of basic current mirror circuit  we are concentrating at half left side of  circuit which involves Q1 transistor and is in active region.

as we can see in the above figure collector and base terminal are short-circuited and the potential at collector terminal is equal to the potential at the base terminal so the potential difference between collector and base terminal will be zero. i.e   Vcb = Vc - Vb = 0 .

for implementaion of current mirror  B(beta) should be  extremely large or infinite then 

base current Ib = 0 (negligible ), Beta = Ic/Ib = infinite.

now we can write emitter current as 

where Io is reverse saturation current and Vt is thermal voltage.

Emitter current will be equal to Iref  because base current is 0. i.e   Ie = Iref.

Now we will concentrate at half right circuit which involves Q2 transistor and it is in active region.

We want to mirror current so this Q2 transistor should be identical and matched to Q1.

As Ib  is zero then Ic = Ie = Iout.
Now base to emitter potential difference will be given as

Same As we did for Q1 .

As we know transistor Q1 and Q2 are identical and perfectly matched that's why 

VBE1 = VBE2.

Hence after solving the above equation we get Iref = Iout. (It shows that the input current is mirroring at output)

Now if beta is finite then Ib will not be equal to zero it will have some small value.

This   expression is valid for both case beta = infinite and beta = finite.

Requirements for current mirror circuit :

1. Both transistors should be in active region (in active region transistor works as an amplifier)

2. Both transistors should be identical and perfectly matched.

I.e VBE1 = VBE2 

         IB1 = IB2

     beta1 = beta2

3.Junction area of both transistors should be identical .

                     Ae1 = Ae2