Both 1/f noise and white noise appear at the inputs of an op-amp and are defined as equivalent input noise voltage. The equivalent input noise is amplified by a gain and appears at the output. In particular, care is required as to low-frequency noise because its voltage is dependent on frequency.
To amplify a tiny signal, multiple amplifiers are sometimes connected in a cascade in order to prevent abnormal oscillation. Let’s consider how each amplifier stage affects the noise that appears at the output of the cascade amplifier.
Figure 3-11 shows a three-stage cascade amplifier.
As shown in Figure 3-11, the output signal power (PSout3) and the output noise power (PNout3) can be calculated as follows.
As you see, the input noise (PNin) and the equivalent input noise (PN1) of the first-stage amplifier have the greatest impact on the output noise.
The output signal power (PSout3) and the output noise power (PNout3) are expressed by the following equations:
PSout3 = G1 × G2 × G3 × PSin
PNout3 = G1 × G2 × G3 × (PNin+PN1) + G2 × G3 × PN2 + G3 × PN3
Therefore, the noise factor (F), a measure of noise, is calculated as follows: