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# MOS Problems intuitively

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#### Saransh22

##### Newbie level 5
Please provide tips how to solve MOS problems for finding voltage gain and output Resistance. Like not by following small signal model but more intuitively and applying direct voltage and current formula on points.

Intuition is a result of knowledge and experience. There is no shortcut.
You need to solve several circuits and find how they are working. Then you gain intuition.

To apply intuition you need to fully understand how a MOSFET works and what it is doing in a circuit.
Once you have that, then intuition will follow.

Most of the intuition you are asking for does revolve around small signal modeling.

As a linear amplifier, the MOSTFET operates as a transconductance device (gm) so small signal voltage in gives small signal current out (iout=gm*vin). If you want to have a voltage out, you provide some load resistance that the current is applied across. So, vo=(iout*rout) = (gm*vin)*rout. That's a basic MOSFET amplifier in a nutshell.

The different types of rout (as others mentioned) comes with experience. Some common examples are fixed passive resistance, diode connected load FET (1/gm or low impedance node), gate biased load FET (1/gds high impedance node), stacked or feedback configurations (even higher impedance). If several of these types are connected to one node, the lowest impedance will dominate. This is how designers quickly try to look for dominant/non-dominant pole nodes. All of which are usually analyzed and derived by looking at small signal ac analysis and configurations.

It's understanding the small signal analysis (along with experience) that gives you much of the intuition.

Saransh22

### Saransh22

Points: 1
Definitely got a concept from your explanation.
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