Design of a simple Constant Current Source Circuit in Cadence Viruoso:

A simple current source can be realized by using a current mirror circuit.

A current mirror consists of 2 NMOS (M1 and M2) placed back to back the drain gate terminal connected for M1. This provides a feedback path to the current and thus keeps the current in M1 constant.
A resistance R1 is connected between the drain of M1 and VDD. We fix the value of primary current (input current ) by adjusting the value of R1.

So for a given inpur current I,
R1(approx)= (VDD-Vgs)/I
Once R1 is determined, Connect the drain terminal of M2 to a voltage source V2 and sweep it from 0 to 1.8V (using DC analysis , for 1.8um technology).
Plot the graph of Vgs vs V2 and determine the region where saturation occurs.
Now keeping the value of V2 in the region such that all the transistors are in saturation, vary the W/L of the MOSFETs.
It will be found that in such case our current through M2 will be directly proportional to the (W/L)of M2 wrt W/L of N1.
keeping the W/L of M1 fixed , vary the W/L of M2. The current through M2 will also vary in the same manner.
I1/I2 = (W/L)1/(W/L)2
It should be noted that theres a particular current range for which this relation holds good and it doesn't beyond this range.
APPLICATION OF A CURRENT SOURCE IN A SIMPLE TWO STAGE OP AMP (designed using 0.18um technology)

here we have used a cascoded current mirror as current source.