"Chapter 1 - The Op Amp's Place in the World" - HTL Wien 10

Vn
Vp
Q1 Q2
INPUT
STAGE
Figure 11–8.Figure 7. Simplified Op Amp Schematic
Q3
11.3.13 Equivalent Input Noise
11.3.13.1 Spot Noise
Q4
Noise is covered in more detail in Chapter 10.
CC
SECOND
STAGE
Q5
Additional Parameter Information
D1
D2
OUTPUT
STAGE
Q6
Q7
Understanding Op Amp Parameters
+VCC
VO
–VCC
All op amps have parasitic internal noise sources. Noise is measured at the output of an
op amp, and referenced back to the input. Therefore, it is called equivalent input noise.
Equivalent input noise parameters are usually specified as voltage, V n, (or current, I n) per
root Hertz. For audio frequency op amps, a graph is usually included to show the noise
over the audio band.
The spectral density of noise in op amps has a pink and a white noise component. Pink
noise is inversely proportional to frequency and is usually only significant at low frequencies.
White noise is spectrally flat. Figure 11–9 shows a typical graph of op amp equivalent
input noise.
Usually spot noise is specified at two frequencies. The first frequency is usually 10 Hz
where the noise exhibits a 1/f spectral density. The second frequency is typically 1 kHz
11-17