- #1

zenterix

- 622

- 81

- Homework Statement
- I will show below the S circuit model of the MOSFET inverter. It contains a resistor in it. I'd like to know why that resistor is needed.

- Relevant Equations
- It seems to me that the inverter would work the same without the resistor.

Here is a MOSFET-based inverter combinational gate

which we represent with the symbol

The MOSFET in the first diagram is itself an abstraction for another circuit (the S model of the MOSFET), which is as follows

The MOSFET is a nonlinear circuit but the two sub-circuits above are linear and represent the ON and OFF states of the MOSFET.

Now, if we put this S model into the first diagram above, then we get the "S circuit model of the MOSFET inverter"

My question is about the presence of the resistor.

The book I am reading says

In the case of the MOSFET being OFF, wouldn't ##v_{OUT}## be ##V_S##?

Is this not higher than when the resistor is present?

which we represent with the symbol

The MOSFET in the first diagram is itself an abstraction for another circuit (the S model of the MOSFET), which is as follows

The MOSFET is a nonlinear circuit but the two sub-circuits above are linear and represent the ON and OFF states of the MOSFET.

Now, if we put this S model into the first diagram above, then we get the "S circuit model of the MOSFET inverter"

My question is about the presence of the resistor.

The book I am reading says

When ##v_{IN}## is high, the MOSFET is in the ON state (assuming that the high voltage level is above the threshold ##V_T##), thereby pulling the output voltage to a low value. In contrast, when the input is low, the MOSFET is off, and the output is raised to a high value by ##R_L##. Here we see the purpose of the load resistor ##R_L## - it provides a logical 1 output when the MOSFET is off. Futhermore, ##R_L## is chosen to be large so that the current is limited when the MOSFET is on.

In the case of the MOSFET being OFF, wouldn't ##v_{OUT}## be ##V_S##?

Is this not higher than when the resistor is present?