Electronics > Beginners
Issue interpreting AC phase plot
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Ian.M:
Scaling off the low frequency gain from the plot I get about 31dB, which is a voltage gain of about x 36.  The feedback resistor is effectively divided by (1+gain),   giving an effective input resistance of about 37K.   That's not quite consistent with your input impedance plot, but is no more than one pixel out, which could be rounding errors in the plot.  Use a cursor and check both the gain and the input impedance properly!

You do make this hard on us by just posting screenshots, without even using measurement cursors on the waveforms.  Next time if you want a quicker better answer  attach the sim .asc file.  If it needs any 3rd party symbols or libraries, zip them up with the asc file, so its ready to run.  If it needs 3rd party models you have added to the LTspice standard model files, you need too identify them and place them as dot commands on the schematic.

promach:
See the attached asc file and mosfet library
Ian.M:
Yep.  Changing the gain plot y axis to linear for easy measurement and using cursors, I get gain=21.36103 and input impedance=44.720659Kohm, both at 1Hz, which are consistant with the formula I posted above.

If you are having difficulty understanding it, divide the feedback resistor into two series parts, one of 44720.659 ohms connected to the input and the other of 955279.341 ohms connected to the output, and consider the voltage on the node in the middle!

Hint: you can share your waveform view by saving the plot settings and including the .plt file in the zip with the sim.  Don't include .raw files as they get very big and are regenerated on every run.
promach:
Rin = R1 / (| Gain | +1). At higher frequencies, the output capacitor reduces the gain and Rin -> R1.

but why Rin drops again after 2MHz ?
Ian.M:
You'd have to do a detailed mathematical analysis including modelling the MOSFETS to be certain, but  I'd certainly expect a dropoff with increasing frequency due to the gate capacitance. 
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