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# Need help on forward body biasing!!

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#### jatan1

##### Newbie level 3
Hi fellow members!

i need help on the topic of forward body biasing.

I am currently using the 65nm process with a wp/wn ratio of 240/120 which is 2.

In regards to the body of the pmos and nmos, i normally tie them to VDD and GND respectively. However, coming upon the topic of forward body biasing i am quite confused as to how should i implement the technique? Could anyone help me out? Thanks!

My goal is to simulate my circuit in ultra low power which Vdd as low as 0.2V to 0.4V. Would appreciate any form of help.

help anyone?

I am not a fan of this. Yes, you can lower the "effective VT"
by about 0.6V/V. But you also are forward-biasing the
parasitic BJT base, and even if you don't "quite" get to
elevated DC drain current you are pushing further into
substrate noise amplification.

Now with that low a VDD, maybe you can't light it up
and don't care.

I would suggest using a replica feedback bias scheme
to servo body voltage to "what you want from the FET".
If it's zero VT(1uA) then you put a G=0, S=0, B=amp
output, D w/ 1uA load = amp feedback replica device
in a control loop and hope for chip-wide matching.

But if you don't need that last couple octaves of speed,
comes, at much less complexity / risk.

I am not a fan of this. Yes, you can lower the "effective VT"
by about 0.6V/V. But you also are forward-biasing the
parasitic BJT base, and even if you don't "quite" get to
elevated DC drain current you are pushing further into
substrate noise amplification.

Now with that low a VDD, maybe you can't light it up
and don't care.

I would suggest using a replica feedback bias scheme
to servo body voltage to "what you want from the FET".
If it's zero VT(1uA) then you put a G=0, S=0, B=amp
output, D w/ 1uA load = amp feedback replica device
in a control loop and hope for chip-wide matching.

But if you don't need that last couple octaves of speed,
comes, at much less complexity / risk.

i realize in some circuits, simply apply a "0" to the pmos body and this provides FBB. Dont seem to understand why though.

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