Consider the amplifier when used as an inverting amplifier (Figure
1.43), i.e. an amplifier which gives an output which is out-of-phase with
respect to the input. For the circuit shown in Figure 1.43, the
connections for the power supply and the offset null have been omitted.
The input is connected to the inverting input, the non-inverting input
being connected to earth. A feedback loop is connected, via the resistor
R2, to the inverting input. The output voltage of such an amplifier is
limited to about • V and thus, since the gain is about 100 000, the
input voltage to the inverting input at X, Vx, must be between about
+0.0001 V and-0.0001 V. This is virtually zero and so point X is at
virtually earth potential. For this reason it is called a virtual earth. The
potential difference across the input resistance Rl is (Vm - Vx) and thus
(Vm - Vx) = l~Rl. But Vx is virtually zero and so we can write: