When the op amp is not saturated, the inverting amplifier produces an output voltage, Vo, that is proportional to the input voltage, Vs. The output voltage is measured using a voltmeter. The input voltage is the voltage of the voltage source.
The output of the inverting amplifier is given by.
In this example, volt sources were used. As a consequence, the op amp saturates at volts (i.e. = 20 volts).