Difference between "1/gm" and "r"

[haideruet]

Hi,
I am Little confused between the two terms , 1/gm and rd
some times when calculating output impedance we use 1/gm and some times we say it is rd.
when diode connected load is considered.
What is the basic story and when each may be considered while designing CMOS circuits.

I am new to CMOS circuits. I read razavi book several times. but I did not find this answer.

Thanks
Hayder

 

[erikl]

For a single (isolated) transistor, I think the drain output impedance rd is identical to 1/gm . In practical circuits, however, transistors are never alone, so - depending on the circuit - rd can also be influenced - independently of 1/gm - by other devices connected to the other nodes of the MOSFET, e.g. by the presence of a source series resistor.

I hope somebody can explain this in more detail - with an equation.