* In a broad sense, a virtual ground is a node of a circuit that is maintained at a steady reference potential without being connected directly to a power supply or ground. In the circuit of Figure 2-16, VIN(-) is called a virtual ground since it is virtually equal to GND.
Next, let’s calculate the closed-loop gain (AV) of the noninverting amplifier shown in Figure 2-17 using a virtual short and the ideal op-amp. Let’s express the output voltage (Vo) as a function of Vi. From the concept of a virtual short, VIN(-) = VIN(+) = Vi.
Therefore, the current flowing through R1 (i1) is calculated as follows:
I1 = VIN(-) / R1 = Vi / R1
No current flows to the op-amp input since it has infinite impedance. Letting the current flowing through R2 be I2, I1 = I2. Hence, the voltage across R2 (VR2) is:
VR2 = R2 × I2 = R2 × Vi / R1
Hence, Vo is calculated as:
Vo ＝ VR1 + VR2
= Vi + R2 × Vi /R1 = Vi × (R1 + R2) / R1
AV = Vo / Vi = (R1 + R2) / R1
You can easily find the closed-loop gain equation.
The closed-loop gain (AV) of the inverting amplifier shown in Figure 2-18 can also be calculated in the same manner.
VIN(-) = VIN(+) = 0 V (GND)
I1 = V1 / R1 = I2
Vo = VR2 = R2 × I2 = R2 × V1 / R1
Hence, the closed-loop gain is:
AV = Vo / Vi = R2 / R1
As described above, the closed-loop gain can be calculated easily using the concepts of a virtual short and the ideal op-amp.