Analog IC Design problem...urgent..help needed..

The current source is to be designed to work with a 5 volt power supply. The current is to be set with an external resistor of value 100-500K. The 100K ohm resistor is to correspond to (about) 10 micro amps and the 500K to (about) 2 micro amps. The output resistance of the current source is specified at over 50 M. The output must function over the range of 1.4 to 5.0 volts. Finally, it is specified to have less than an 8% peak variance with a 2.5 to 5.0 volt impulse on the output side.

Design your circuit using the 1 micron models.

can any one gimme the idea of what I have to do???
If you want the models I can post them.

any body help me out..plz

If you start with a simplest CMOS cascoded current source (that is 2 diode drops to set the reference and then mirror the bias and cascode voltage). you can then calculate what the Vgs drop required is in order for 10uA to flow in the 100k ohm case (or 2 uA to flow in the 500kohms case). for e.g.

(Vcc - Vx)/100k = 10uA; where Vx is the drop of the diode connected reference device. So, if you start with a simple cascoded current source you have 2Vgs = 4V from the above equation.
Adjust your W/L so that you get about 4V across the 2 MOS diodes. then you can connect the mirrors.
you can the measure your output impedance and see if it > 50M. If not, adjust your lengths of your current mirrors to get that impedance (while keeping your W/L constant as you dont want to change the 2Vgs = 4V condition)

hope this helps.

can you send me the circuit diagram plz...

check page 5 of:
**broken link removed**

If your VIN does not vary, a folded cascode mirror wil work fine. Just remember if you VIN changes, so will your current.

You may be better off designing a bandgap, a unity gain op amp and generating a Vbg/R current source then feeding that into your current mirror. THat will make your current much less sensitive to temperature, process, and input voltage.

Thanks transbrother..
i will simulate it and i will let you know if i have any future problems..i think i can manage now..but i am not so sure..
any ways thanks a lot..