Solve PMOS Transistor Circuit: Vthreshold, Vgs, Vgt, Vds

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SUMMARY

The discussion focuses on determining the operating region of a PMOS transistor circuit with specified parameters: Vthreshold = -0.4 V, Vgs = -3 V, Vgt = -2.6 V, and Vds = -2.6 V. The user confirms that the transistor is not in the Cutoff region since Vgs is less than Vthreshold. To ascertain whether the PMOS is in the Linear or Saturation region, the user must evaluate the relationship between Vds and Vgt. The conversation highlights the importance of understanding the voltage relationships in MOSFETs, particularly Vgs = Vgd + Vds.

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  • Basic circuit analysis involving resistors and voltage sources
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Maxwell
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I have a PMOS transistor circuit set up.

These are my parameters:

Vthreshold = -0.4 V
Vgs = -3 V

Vgt = Vgs - Vthreshold = -3 + 0.4 = -2.6 V

Vds = Vgs - Vthreshold = -2.6V

I'm trying to figure out what operating region this stupid thing is in. There are so many different ways of figuring this out, that it's getting very confusing -- it seems every source uses a different notation.

I know it's not in the Cutoff region, because Vgs < Vthreshold. However, since Vds and Vgt are the same, how do I decide if it is in Linear or Saturation region?

For linear: Vgs < Vthreshold and Vds > Vgt

For saturation: Vgs < Vthreshold and Vds < Vgt

This is really starting to bug me.

If I could figure out how to find Vgd -- that is, voltage from gate to drain, I might be able to figure something out. However, my Vin = 2V and the drain is connected to a variable resistor and then to ground, I think Vgd = 2V. But I don't think this is possible since it's a PMOS, so I'm ignoring that parameter for now.

Thank you.
 
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what is Vgt? never have come across this...
The voltages around MOSFET usually have this relation Vgs = Vgd + Vds. So, I don't think that you can get Vds the way you did.
To determine the region of operation you need to look at inequality of this sort: Vds ? (Vgs - Vtp)
However Vds is not given... so you'd either have to know extra parameters like Id, W/L or assume a value/region and then verify.
 

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