The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> RF Design >> VCO tail inductor induces B-S forward biasing
https://designers-guide.org/forum/YaBB.pl?num=1268901114

Message started by rfmems on Mar 18^th, 2010, 1:31am

Title: VCO tail inductor induces B-S forward biasing
Post by rfmems on 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.

Title: Re: VCO tail inductor induces B-S forward biasing
Post by aaron_do on 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...

Title: Re: VCO tail inductor induces B-S forward biasing
Post by rfmems on 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.

Title: Re: VCO tail inductor induces B-S forward biasing
Post by aaron_do on 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...

Title: Re: VCO tail inductor induces B-S forward biasing
Post by rfmems on 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.