Designer’s Guide Community

‹ Previous topic | Next topic ›

Pages: 1

VCO tail inductor induces B-S forward biasing (Read 3400 times)

rfmems

Senior Member

Offline

Posts: 121

VCO tail inductor induces B-S forward biasing
Mar 18^th, 2010, 1:31am

If I add tail inductor (noise filtering) to an LC cross-coupled NMOS VCO, the 2nd order harmonic swing on the tail node will forward bias the cross-coupled NMOS bulk source diode, if the bulk is connected to ground.

Is this going to be a problem? I have seen numerous paper on that structure but nothing mentioned on that issue.

IP Logged

aaron_do Senior Fellow Offline Posts: 1398	Re: VCO tail inductor induces B-S forward biasing Reply #1 - Mar 18^th, 2010, 6:42am That's an interesting observation. I doubt it is a problem though. I don't think it is enough to turn on the diode in practice. I haven't seen any works talking about it either...
Back to top	there is no energy in matter other than that received from the environment - Nikola Tesla IP Logged

rfmems Senior Member Offline Posts: 121	Re: VCO tail inductor induces B-S forward biasing Reply #2 - Mar 18^th, 2010, 8:27am aaron_do wrote on Mar 18^th, 2010, 6:42am: I don't think it is enough to turn on the diode in practice. As soon as the diode is forward biased, there will be current flowing. And if the mosfet used is low vt high speed, the current can be big. I am not sure it is not a problem in practice.
Back to top	IP Logged

aaron_do Senior Fellow Offline Posts: 1398	Re: VCO tail inductor induces B-S forward biasing Reply #3 - Mar 18^th, 2010, 8:01pm Have you actually simulated the current and how big it is? It seems to me that that particular node could be grounded (at the second harmonic) anyway without much affecting the performance...
Back to top	there is no energy in matter other than that received from the environment - Nikola Tesla IP Logged

rfmems Senior Member Offline Posts: 121	Re: VCO tail inductor induces B-S forward biasing Reply #4 - Mar 19^th, 2010, 1:00am Bulk-source forward biasing is not modelled in my design kit. I did some rough calculation, it can reach several mAs peak. If that node is shorted, it can still work, but with worse performance, since the tail inductor/noise filtering is not there anymore.
Back to top	IP Logged

Pages: 1

‹ Previous topic | Next topic ›