Diode connected transistor as voltage clamp.

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sys_eng

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On the first one, it's pretty consistent that a nmos diode connected transistor has drop of 1V across VDS of M13, M12


However, on the second one. VDS of M41 is only 51.87mV, I am expecting 1V. VDS of M40 is 2.3V and I am expecting 1V.


How come there are differences of two pictures showing diode connected transistor has different VDS across.
why?
 

The second schematic snippet is useless because it doesn't show which current is injected to individual transistors.

We can just assume that some diode connected transistors get about no current and thus have low voltage drop.
 

sys_eng said:
How come there are differences of two pictures showing diode connected transistor has different VDS across. why?
Click to expand...

The diode-connected Vds depends on Ids of the transistor - as suggested above -, s. the foll. sample simulation:


For bigger Ids currents, the diode-connected Vds more and more approaches the SPICE-calculated saturation voltage Vdsat.
 

erikl said:
The diode-connected Vds depends on Ids of the transistor - as suggested above -, s. the foll. sample simulation:


For bigger Ids currents, the diode-connected Vds more and more approaches the SPICE-calculated saturation voltage Vdsat.
Click to expand...

vds depends on Ids?
Most textbook shows Ids depends on vds instead. The other way around.
 

FvM said:
Surely, it's just the difference of been constant voltage versus constant current driven.
Click to expand...

I am not sure if I understand that statement.
I have never seen that graph Ids on x axis and vds on y axis on any text book.
 

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