@User I usually break the loop with a 0 hertz lowpass filter (in this case a 10G Ohm resistor + 1Farad capacitor) so that for any frequency above 0 hertz the loop appears as open, while it is closed for dc operating point calculation. In the example (an amplifier in signal follower configuration, where the output is fed back to the inverting input) i have also restored the gate capacitance loading effect on the output, since after breaking the loop this load would be missing. You see also the placement of the AC signal source.