When to use a dioded connected transistor(PMOS/NMOS)?

[nozombie]

Hi there,

I'd like to know what is the benefit of using a dioded connected transistor, I always see them in pull up/down circuits.
Thanks

 

[dick_freebird]

Depending on geometry, they may be just the cheapest
possible resistor-like element (here you'd see very long
L, near minimum W). Or sometimes, you just want a
"diode" that is not pinned to one rail or the other, at
one end or the other (well diode, not too useful except
for one side of an ESD clamp).

I don't work on stuff that's that cost sensitive so I will
use a process with a richer palette and whatever
element does the job -best-, not cheapest. But other
folks have fewer options, and you get to ride the horse
(or donkey) you're on.

 

[erikl]

The main advantage of a diode-connected transistor IMHO is the assured operation in saturation mode (V_DS > V_DSsat , s. the PDF below), so guaranteeing a decently large dynamic resistance as a load. View attachment diode-connection.pdf

... sometimes, you just want a "diode" that is not pinned to one rail or the other,
at one end or the other (well diode, not too useful except for one side of an ESD clamp).

Actually it's an antiparallel diode if a MOSFET is used - unless you manage to switch the bulk connection depending on the voltage direction.