# Input impedance of common-gate stage in CMOS

Consider the following common-gate stage designed in CMOS:

## Calculating input impedance from small-signal model

If we replace the circuit with it's small-signal equivalent and then apply a voltage source Vx and measure current Ix with body effect included, we obtain the following:

## Calculating input impedance just be inpsection (intuitively) in the original circuit

I know it isn't usually done like this and that input impedance is almost always defined in small-signal.

However, just by looking at the circuit and thinking if I applied an increase voltage step at the source of M1, and considering that if the source increases, then VBS (body-source voltage assuming body is 0V) decreases by becoming more negative and VGS (gate-source voltage) decreases, hence you would think that the current decreases as well just by the square-law equations.

So, an increase voltage at source of M1 has led to a decrease in current, this would lead me to assume that the circuit has a negative input impedance looking into the source.

Why is this not consistent with the first approach?