NMOS Bulk connections - Source -Bulk voltage

[shaikss]

Hi,

Rf voltage is fed as input to NMOS source. RF input has both positive and negative voltage.
Bulk of NMOS is connected to most negative voltage in the complete circuit.
Now, the RF voltage has negative component.
So, how should I connect the bulk of NMOS?

How will the positive VSB(source- Bulk) and negative VSB (Source-bulk) impact?
What and how should be the bulk current for NMOS?

RF input is fed to source of NMOS and gate is being fed by control voltage of 1V. Bulk is grounded. Now, when I check the VSB, its has both positive and negative cycles. What is the impact of this behavior on the overall design? When I checked the bulk current, it is around 400uA when the width of MOS is 40um.

Pls help me out.


Note: All discussion is related to NMOS.

 

[FvM]

I see that you are asking at Edaboard about MOSFET bulk connections since about 18 months.

I presume you know that source and drain are isolated from substrate by a junction. The "impact" of forward biasing a substrate junction should be quite obvious.

To avoid substrate currents, the source or drain should not swing below junction potential by e.g. 0.3 V. IN a rectifier circuit, the NMOS substrate could be e.g. connected to negative DC output.

 

[shaikss]

I see that you are asking at Edaboard about MOSFET bulk connections since about 18 months.

I presume you know that source and drain are isolated from substrate by a junction. The "impact" of forward biasing a substrate junction should be quite obvious.

To avoid substrate currents, the source or drain should not swing below junction potential by e.g. 0.3 V. IN a rectifier circuit, the NMOS substrate could be e.g. connected to negative DC output.

The only bothering thing to me is that the negative peak is always lower than the negative DC voltage. So, II order body effects still exists. Since it is time varying input signal and if reference point is not fixed, then the RF input signal has its reference to any voltage. In those aspects, it will be difficult to understand to which voltage our bulk should be tied to avoid body effects.

 

[kenambo]

Hi,

It is no matter you connect bulk with most negative voltage..

Because in Nwell process....the substrate is P substrate .. So it is better to give low potential to substrate and it is easy to do layout...

thanks

 

[FvM]

The question is, does the negative voltage exceed diode forward voltage, respectively do you drive considerable current into the substrate diodes. But I don't think that there are much alternatives than connecting the substrate to DC-.