Expressing Operating point of a NMOSFET

Audioguru said:

It is difficult to calculate the values for R1, R2 and R3 because a Mosfet has a range of gate-source voltage for a certain current and a range of current for a certain gate-source voltage.
The range of voltage gain of a Mosfet complicates the calculations.

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Does it get easier if we assume that the MOSFET is active in it's operation point?

Also (in the first circuit) since there is no current through R1, we can assume that the voltage at the gate is also VDD.

Teszla said:

Does it get easier if we assume that the MOSFET is active in it's operation point?

Also (in the first circuit) since there is no current through R1, we can assume that the voltage at the gate is also VDD.

Click to expand...

If the gate of a Mosfet is at VDD and the source is near 0V then the Mosfet is turned on as hard as it can go and will not amplify and will not pass a signal.

The gate voltage must be at a suitable voltage so that the drain resistor has a voltage across it but this voltage can increase and decrease when there is a signal fed to the Mosfet amplifier. Mosfets even if they have the same part number have different "suitable gate voltages". Also, each Mosfet has a different amount of voltage gain that changes the operating point.

If the gate of a Mosfet is at VDD and the source is near 0V then the Mosfet is turned on as hard as it can go and will not amplify and will not pass a signal.

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Not necessarily, depends on Vth, Vdd, R2 and R3 values. The available output voltage swing will be < Vth, but that may be appropriate for low Vdd, e.g. 3 - 5 V.