RF Electronics Chapter 10: Operational Amplifiers Page 351
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
needs to be balanced with the probability of the input being damaged and any reduction
in performance caused by the inclusion of the protection devices. For many applications,
the best option may be to design the front end, to enable easy replacement of the front-
end devices if they are damaged by input surges.
Low Noise Designs using Operational Amplifiers
The noise voltage associated with a resistor is;
��
������� Eqn. 10.12
Where:
K = Boltzman's constant 1.38064852 x 10
-23
Joules/Kelvin
T = Absolute temperature Kelvin
B = Bandwidth in Hz
R = Resistance in
In a circuit each resistor is then associated with its own noise source which has a random
noise voltage of E
NR
. In addition the operational amplifier produces noise which can most
accurately be represented as voltage (e
ni
) and current noise sources (i
ni+
and i
ni-
) at the
inputs, as shown in figure 10.14.
Figure 10.14 Noise sources in an operational amplifier configuration (Fig 29 of [4])
Table 10.2. Low Noise RF Operational Amplifiers.
Manuf. Part Type Vs
nV/Hz pA/Hz
1/f kHz BW MHz
TI LMH6629 VFB 5 0.69 2.6 5 900
TI OPA847 VFB 5 0.85 2.7 1 3500
TI THS4022 VFB ±5 to ±15 1.5 2 1.5 350
Anal Dev AD8000 CFB 4.5 to 12 4.3 26- 3.4+ 0.3 1500
Anal Dev AD8099 VFB 5 to 12 0.95 2.6 10 510
Anal Dev ADA4898 VFB ±5 to ±16 0.9 2.4 0.02 65
The values for e
ni,
i
ni+
and i
ni-
are normally obtained from manufacturer's data sheets.
Table 10.2 shows the noise performance of some low noise amplifiers from different
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