Current mirror with FETs

[srinivasbakki]

Hi,
Iam trying to understand how current mirror with FET works. I read on wikipedia that Id is a function of Vdg and Vgs, sure enough. and Keeping Vdg = 0 and driving a current with current source shall set the Vgs. This part is confusing - unless you have Vgs how can the mosfet even allow Id ?
As i Understand - the channel of the mosfet allows current only if there's a Vgs enough to widen the channel and Vdg helps in sucking the current.

Current mirror with BJTs is clear: the collector resistor connected to the base drives base current to bias the transistor and thus sets Vbe.

Can somebody explain current mirror with mosfets ?

Thanks,
Srinivas

 

[Tahmid]

At start-up, VGS increases from 0 to the required final value to pass the required Id.

broken link removed

 

[srinivasbakki]

Well, i understand Vgs kicks off to the required value to drive the current. But what's the trigger for Vgs to increase ?

 

[Tahmid]

If you see from the circuit I attached, there, the gate charges through R1 from the supply. This causes Vgs to increase since the gate is being charged.