how to realize a current source

Hi,

I am developing a charge pump PLL. If I am using the charge pump circuit in Razavi's book. how can I realize the two gate voltage of the current sources? Shall I use voltage reference?

Thank you.

use a bias current generator
i.e. current is generated from bias cell then mirrored to the CP

Thanks.

Could you give more detail about the bias current generator?

NO, gate voltage is detemined by the Id of it. You should design current bisa. After you have a constant (some range) current, use current mirror as the current source.