Operational amplifiers

o takes three nodes, in this order:

o1,n+,n−,nout

No power rails, no supply, no gain figure. It is an ideal op amp: a nullor, which is the statement that the two inputs are at the same potential and draw no current, and that the output supplies whatever the rest of the circuit needs.

An inverting amplifier: a 2 V source into the op amp's inverting input through 10 kΩ, a 47 kΩ resistor feeding the output back to that input, and the non-inverting input on ground. We name the source e and the resistors r1 and r2, call the summing junction m and the output o, and write the op amp's three nodes in the order the element wants, non-inverting input first:

e,1,0,2
r1,1,m,10'k
r2,m,o,47'k
o1,0,m,o

Run in DC, the output is

That is −9.4 V from 2 V, a gain of −4.7, which is −47k/10k, arrived at from the nullor conditions rather than from the formula.

Note the node order — 0 is the non-inverting input and m the inverting one. Swap them and the feedback becomes positive, which is a different circuit and will usually refuse to solve.

What the op amp reports

An o reports a current and a power, and no voltage drop — its input terminals are at the same potential by definition, so a drop across it would not mean anything. The current is the one it sources or sinks at its output.

When ideal is not enough

There is no finite-gain op amp element. Model one as a dependent source, which is what a finite-gain op amp is. The same amplifier, with the o element replaced by a voltage source we call ea, driving the output node with 100,000 times the voltage between the two inputs — the non-inverting one, at ground, minus the inverting one at m:

e,1,0,2
r1,1,m,10'k
r2,m,o,47'k
ea,o,0,100000*(0-vm)

Run in DC, the output is now

which is −9.39946 V against the ideal −9.4 V: the two differ in the fifth figure, which is the honest way to find out whether the ideal model was good enough for your problem.

The same shape gives you finite input impedance (a resistor across the inputs) and non-zero output impedance (a resistor in series with ea). Once you are modelling it, the op amp is just circuit elements.