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Design >> Analog Design >> voltage controlled current source
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Message started by nandu on Jun 21^st, 2006, 3:05am

Title: voltage controlled current source
Post by nandu on Jun 21^st, 2006, 3:05am

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)?

Title: Re: voltage controlled current source
Post by amarnathdinamani on Jun 21^st, 2006, 10:40pm

Hi , for desiggning a VCO u need to change the bias current of each of the inverters, i.e u need to vary the charging /discharging time.U can do this by changing the Vgs of an input transistor and then mirroring this to the inverter chains.Refer baker and boyce.

amarnath

Title: Re: voltage controlled current source
Post by nandu on Jun 22^nd, 2006, 12:01am

yes in this case i am varying the charging/discharging time by varying the current. the trouble is if i yank the vgs of a mos and mirror the current, the current varies by a huge amount for a very small voltage range (about 0.1V only) and then the mos goes to triode. So i get an unreasonable large voltage-to-current gain for an unreasonably small input voltage range. what im looking for is a VCCS with a moderate gain over a large range of voltage (say 1V).

Title: Re: voltage controlled current source
Post by mikki33 on Jun 27^th, 2006, 6:21am

So, try to use a sum of constant current and voltage controlled current. Transistors should have very large L.
Make sure, the VCO runs at lower frequency vs. PVT when VC=VDD (Your work point is in the middle of the VC range and you have enough margin for post-layout)

Title: Re: voltage controlled current source
Post by mikki33 on Jun 27^th, 2006, 6:22am

And what you have to see

Title: Re: voltage controlled current source
Post by mikki33 on Jun 30^th, 2006, 1:09am

Dear nandu,

you asked the question. Did you see the answer?

Title: Re: voltage controlled current source
Post by kayathi on Jul 10^th, 2006, 3:27pm

Just a thought, did you try source degeneration on your transistor? - I guess it would help reducing the change of current with change in vgs.

-kayathi

Title: Re: voltage controlled current source
Post by mikki33 on Jul 11^th, 2006, 2:57am

No, I didn't use it. And even didn't think about it...