One problem about opa ac. analyse

To simulate one opa, ac. analyse is needed. What does the frequency indicate when we mention frequency scan in ac. analyse? What difference between input signal frequency and this frequency?
Thx!

Any simulator will solve a number of ordinary diferential equations (ODEs) given by Kirckoff laws over all nodes. Those equations can be writen in time (Transient analysys) or in frequency (AC analysys)

Because solutions for both .TRAN and .AC depend on initial conditions (IC), those IC will be found first, to have an initial point to start solving the equations(DC analysys).

Because frequency equations are very complicated (those H(j*Omega)), a linearization of the Time ODEs around IC is performed first.

Then a "small" sinusoidal signal (whith variable frequency) is aplied at entry to observe the output.

This is how .AC analysys is performed

Because a linearisation was performed first, there is no diference between input and output frequency in .AC analysys

When the amp is used as OSC ,the input signal frequency is the .ac working frequency.
When the amp is used as following, the input signal
frequency is 0, the .ac analysis considers only stability.
I hope that helps!