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Fundamental Noise Basics and Calculations

Fundamental Noise Basics and Calculations 53

Voltage noise density e n
Current noise density i n
e
n
i n

Figure 3.4 Equivalent
noise sources of an opera-
tional ampliﬁer.

TABLE 3.4 Noise Characteristics of Typical Bipolar
and FET Opamps
Typical bipolar opamp (e.g., OP07)
Input bias current (i b) 10nA
Input offset voltage (V os ) 0.25mV
Input voltage noise density (e n) 12nV/ Hz
Input current noise density (i n ) 0.2pA/ Hz
Typical FET opamp (e.g., LMC7101)
Input bias current (i b) 64pA
Input offset voltage (V os) 10mV
Input voltage noise density (e n ) 37nV/ Hz
Input current noise density (i n) 1.5fA/ Hz

3.7 (Operational) Ampliﬁer
Equivalent Noise Sources

Whether constructed from discrete components or opamps all ampliﬁer systems
can be described in terms of two frequency-dependent noise sources, a voltage
noise generator e n and a current noise generator i n (Fig. 3.4). Both these quan-
tities are usually speciﬁed in manufacturers’ data sheets for opamps and are
either speciﬁed or can be calculated from discrete transistor data sheets. The
current noise generator is essentially just the shot noise of the bias currents
in the ampliﬁer input stages. Depending on bias current there are large differ-
ences in current noise spectral densities among opamps, especially between
bipolar devices, which run at relatively high base bias currents, and FET types,
which require much smaller gate bias currents. Table 3.4 compares the noise
generators of a typical bipolar and an FET opamp. Current noise is clearly much
higher in the bipolar device than in the FET device, whereas voltage noise
is somewhat smaller. Nevertheless, both these devices can be labeled “low
noise.” Whether one or the other performs better in your measurement is a
function of the circuit conﬁguration. Another important difference to note
between bipolar and FET types is that FET bias currents, and with them
current noise magnitudes, are very dependent on temperature. Hence if high
operating temperatures are expected, these parameters in particular should

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