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MOSFET operating region
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santoshgurral:
Hi

I am working on a DC electronic load circuit. Attached here is my circuit. I want to understand what region does the MOSFET operates, ohmic or saturation region when used as DC electronic load?
How do I identify in my circuit that MOSFET is in the ohmic or saturation region? In general, what are the application where MOSFETs operated in the ohmic region and saturation region specifically?

Thanks
Santosh

Fire Doger:
Ohmic

You can either calculate/simulate/measure the voltage on the gate and check it's datasheet or have a look on the circuit around it.
For example in your schematic there is only a 2m resistor between S and GND, if you drive it in saturation you have a source (pretty much) shorted to ground  :-// Do you think that it will work in saturation?
If it was driving a load (led, motor, etc) then the chances of being in saturation increases.

Ohmic is used when a voltage controlled resistor is needed. ie amplifier circuits, load circuits etc...
Saturation is used when you need a voltage controlled switch. ie driving a 12V line from 3.3 signal. Usually you will have a binary "control signal" (high - low, voltage levels depends on circuit)
duak:
I'm sorry to say that  Fire Doger is not correct.   The problem is caused by the terminology.  Go to wikipedia https://en.wikipedia.org/wiki/MOSFET#Modes_of_operation  or search for MOSFET ohmic region.

Saturation in a bipolar transistor occurs when VBE is greater than VCE and the transistor's collector to emitter conductance is at a maximum.  Saturation for MOSFETs is really Current Saturation where for a given VGS the drain current is relatively constant  for a range of VDS.  This is the region used by linear amplifiers and electronic loads.

The Ohmic region is where the MOSFET acts like a resistor, where the resistance is a function of the VGS so that the drain current is proportional to the VDS.  This is also called the Linear region but this region is used in power switching circuits.  Honest!
Wimberleytech:
Saturation: VDS > VGS - VT
Linear: VDS < VGS - VT

If you are schooled in bipolar, the terms Linear and Saturation are kinda reversed for MOS.
Better terminology for MOS is: saturation and non-saturation
magic:

--- Quote from: duak on March 17, 2020, 01:53:46 am ---Saturation for MOSFETs is really Current Saturation where for a given VGS the drain current is relatively constant  for a range of VDS.  This is the region used by linear amplifiers and electronic loads.
--- End quote ---
If you are lucky. At low drain voltage you may end up in the linear legion too.
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