Voltage controlled current source

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nandu

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hi

I intend to design a ring VCO as shown in the figure. Could anyone suggest a voltage controlled current source that i could use to generate the control current (Ictrl)?
 

i tried doing this by changing the vgs of an input transistor and mirroring the current to the inverter stages. but the trouble is that i the current varies non-linearly and by a huge amount for a very small voltage range(of abt 0.2V) and then it moves into triode. so im getting an unreasonable large voltage-to-current gain over a very small input voltage range. i am looking for a VCCS with a moderate gain over a larger input volt range (say 1V). im using 0.18u tech with Vdd of 1.8V
 

i posted a V/I before in this topic , it may work with u.
 

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