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intutitive explanation of the impedance seen at the source of a diode load

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bharadwaj.cv

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In the book "Design of Analog CMOS integrated circuits by Razavi", on page 53 and 54 (section 3.2.2) he says that when looking into the source of a diode connected load the impedance seen at source of M1 is lower when body effect is included. I understand the explanation provided by him using the small signal model, but I am not able to understand intutively how this works. Can some one please explain??
 

This is this way. Drain to Bulk potential another second order effect that creates extra careers and adds up in the conduction.
When you add body effect, Means when your body and source are at different potential, your conduction gets affected called body effect.
For NMOS case, if source is at higher potential than bulk there will be large voltage difference from drain to bulk and hence more carrier. You get extra current that flow from drain to Bulk. In another term bulk potential difference adds up extra resistance, rdb = 1/gmb in parallel to 1/gm . total impedance at source of diode will be (gm+gmb)^-1.
 

There will always be a difference between the drain and the bulk right, whether or not the bulk is grounded??? Did you mean source??? Even then I did not clearly understand.....can you explain in little more detail....
 

When you VDB = VDS, you don't have body effect. Your Vth is decided by already available careers below the gate. But when VDB increases, careers availability increases under gate and adds to Vth.
 

Substrate or body effect is discussed more detailed in paragraph 2.3 As an intuitive explanation, the subtrate acts as a second gate or backgate with it's own transconductance contribution. Because substrate and gate are both grounded AC-wise in the diode load configuration, the total transconductance and in return the source output conductance are both increased.
 

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