What is Current Mirror? | UGC-NET Questions and Answers

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Solve: In the circuit shown below, the voltage at each drain terminal is

Choose the correct option 

 

Answer: Option 1 (See the explanation below)

What is current mirror?

When two transistors are connected thru their source or drain terminals, then current thru each of them will be identical if they are matched or their width (W) and length (L) are same. In this case, two transistors are matched when their width and length are same.  If they mismatches then current flows according to their ratio of width and length to other (transistor). 

Let, assume there are two transistors of width W1 and length L1 and width W2 and length L2, respectively. then, 

i1 / i2 = (W1/L1) / (W2/L2)

where, i1 = current flow thru transistor 1

            i2 = current flow thru transistor 2 

Solution of the above question

We are assuming that both transistors are matched or their width and height are equal. So, current flow thru two transistors are equal.

So, iD1 = iD2 =I/2

From the given circuit, the drain voltage is,

VDD = I/2*RD + VD1

or, VD = VDD - I/2*RD

(VD1 = VD2)