When simulating a circuit consisting of mosfets, when can you say that the MOS should be in cut-off, saturation. or linear (triode) region? Specifically a charge pump circuit utilizing PMOS.(refer to the figure below for the circuit)enter image description here

  • \$\begingroup\$ The datasheet for a MOSFET will tell you the threshold voltage, at which it starts to turn on, and give graphs showing the 'linear region', and maybe full-on. Their Rds(on) is specified at 'full-on'. \$\endgroup\$ – gbulmer Sep 24 '14 at 16:26
  • \$\begingroup\$ Yes, but there are mosfets that act as switch or cases like if its digital, saturating the circuit is not so necessary. Cases like that is my concern.Thanks anyway @gbulmer \$\endgroup\$ – djambalong Sep 24 '14 at 16:33
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    \$\begingroup\$ AFAIK, all MOSFETs have 'off', threshold, 'linear' and saturated regions, described in datasheets. Otherwise the device would be unusable. So go look at the datasheet. It very much depends n what you mean by 'act as a switch'. If a MOSFET is being used in a digital circuit, then it is important to know about the MOSFETs behaviour or specification around saturation. \$\endgroup\$ – gbulmer Sep 24 '14 at 16:40

Every transistor in this schematic is probably operating primarily as a switch. So they all move at some point in time between off and triode. A MOSFET is in saturation if Vds > (Vgs - Vt) (for an NMOS, or |Vds| > |Vgs - Vt| if you prefer for PMOS with Vds, Vgs, Vt < 0). A decent simulator should be able to report the device threshold for biased transistor. Normally we care about maintaining a device in saturation if we want high output impedance, usually when used as a current source or an amplification stage. Charge pumps are essentially switch and capacitor networks for pushing charge around, so the region of operation is less of a concern here.

  • \$\begingroup\$ thank you @mixed_signal. But I am going to make a mathematical model out of it, so is there anything you can advice on how should do it and what important things I should take on to consideration? \$\endgroup\$ – djambalong Oct 5 '14 at 7:51

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