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How Bulk driven MOS works and reduces the threshold voltage?

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simplsoft

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How Bulk driven MOS works and reduces threshold voltage

Hello

I want to ask how a bulk driven transistor works. I have read in some research articles that bulk driven differential pairs is used to operate at low voltages. It reduces trans conductance. But how what happens if input is given to the bulk of the MOS. How it reduces the threshold voltage.
 

Re: How Bulk driven MOS works and reduces threshold voltage

Hi,
The simple model of the threshold voltage of NMOS transistor is given by: Vth0+gamma(sqrt(2phi+VSB)-srt(2Phi)) where VSB is the Source-Bulk potential, Vth0 is the threshold voltage when VSB=0, phi and gamma are technology parameters. We see that if we vary the Bulk potential (VB) the threshold voltage varies, and since the drain current is a function of Vth, it will be modulated by the changes in Vth.

If we want to use Bulk driven MOS as input diffetential stages we must ensure that the transistor is still saturated (VDS>VDS_sat), and that the parasitics diodes don't turn on.

The bulk driven transistors have advantages in terms of low power, but occupy more area (since each transistor needs to be in it's own well).
 

Re: How Bulk driven MOS works and reduces threshold voltage

For a PMOS, In bulk driven, gate is applied with a voltage less than VDD. Lets say the source is at a potential 0.4 V and drain is at let say 0.1V then it means I have to apply a voltage at the bulk greater than the source voltage i.e > 0.4 voltage. If I apply 0.5 V at the bulk so that the parasitic diodes should not be a fwd biased.

It means I have to apply 0.5 V. I apply voltage 0.5V at bulk or at the gate, in both cases 0.5V has to be applied than how bulk driven is used for low voltage application.
 

Re: How Bulk driven MOS works and reduces threshold voltage

"Body effect" reduces VT(eff) by about 0.6V/V (or raises
VT(eff) going negative. You are obviously range limited
by Vf of the B-S diode. Or, you begin to play with the
parasitic BJT as a parallel actor.
 

Re: How Bulk driven MOS works and reduces threshold voltage

It means I can fwd bias the parasitic diode between the body and source?. in bulk driven technique, the current is flowing through the body to source?
 

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