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nandy Junior Member Offline Posts: 29 Timbuktu	voltage controlled current source 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)?
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analog_wiz Junior Member Offline Posts: 31 Universe	Re: voltage controlled current source Reply #1 - 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
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nandy

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Re: voltage controlled current source
Reply #2 - 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).

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mikki33 Community Member Offline Analog/Mixed Signal/High Speed Posts: 57 Israel	Re: voltage controlled current source Reply #3 - 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)
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mikki33 Community Member Offline Analog/Mixed Signal/High Speed Posts: 57 Israel	Re: voltage controlled current source Reply #4 - Jun 27^th, 2006, 6:22am And what you have to see
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mikki33 Community Member Offline Analog/Mixed Signal/High Speed Posts: 57 Israel	Re: voltage controlled current source Reply #5 - Jun 30^th, 2006, 1:09am Dear nandu, you asked the question. Did you see the answer?
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kayathi New Member Offline Posts: 6	Re: voltage controlled current source Reply #6 - 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
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mikki33 Community Member Offline Analog/Mixed Signal/High Speed Posts: 57 Israel	Re: voltage controlled current source Reply #7 - Jul 11^th, 2006, 2:57am No, I didn't use it. And even didn't think about it...
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