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

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htsung1973

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The impedance seen at the source of diode-connected NMOS is lower when body effect is included, from 1/gm to 1/(gm+gmb), is strange for me.
Body effect increases the Vth, so Ids should decrease for same Vgs=Vds(diode connected). That means the impedance should increase, right?
How come it decreases?
 

Vth increases for positive Vsb voltage, which isn't stated in the question. I would expect fixed Ids rather than Vds/Vgs for the diode connected load. Respectively absolute Vgs doesn't matter, only dVgs/dIds for given Ids. "Seen at the source" means you get additional gm produced by dVbs = dVgs.
 

Thanks for your reply.

I am consider a common-source NMOS amplifier with a NMOS diode-connected load to Vdd.

I think body effect is not an assistance to increase gm, since the Vth increases.
The original gm should be reduced by gmb instead of increased by gmb after body effect introduced, right?
If so, the impedance seen from source should be 1/(gm-gmb), not 1/(gm+gmb).
 

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