LMH6654 Datasheet, PDF (14/17 Pages) National Semiconductor (TI) – Single/Dual Low Power, 250 MHz, Low Noise Amplifiers

Application Information (Continued)
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FIGURE 3. Non-Inverting Amplifier Configuration
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FIGURE 5. Non-Inverting Amplifier Noise Model
(1)
R_f||R_g=R_seq for bias current cancellation. Figure 6 illustrates
the equivalent noise model using this assumption. The total
equivalent output voltage noise (e_no) is e_ni * A_V.
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FIGURE 6. Noise Model with R_f||R_g = R_seq
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FIGURE 4. Inverting Amplifier Configuration
Total Input Noise vs. Source Resistance
The noise model for the non-inverting amplifier configuration
showing all noise sources is described in Figure 5. In addi-
tion to the intrinsic input voltage noise (e_n) and current noise
(i_n = i_n+ = i_n−) sources, there also exits thermal voltage noise
associated with each of the external resistors.
Equation 1 provides the general form for total equivalent
input voltage noise density (e_ni). Equation 2 is a simplifica-
tion of Equation 1 that assumes
(2)
If bias current cancellation is not a requirement, then R_f||R_g
does not need to equal R_seq. In this case, according to
Equation 1, R_fR_g should be as low as possible in order to
minimize noise. Results similar to Equation 1 are obtained
for the inverting configuration on Figure 2 if R_seq is replaced
by R_b and R_g is replaced by R_g + R_s. With these substitu-
tions, Equation 1 will yield an e_ni referred to the non-inverting
input. Referring to e_ni to the inverting input is easily accom-
plished by multiplying e_ni by the ration of non-inverting to
inverting gains.
Noise Figure
Noise Figure (NF) is a measure of the noise degradation
caused by an amplifier.
(3)
The noise figure formula is shown is Equation 3. The addi-
tion of a terminating resistor R_T, reduces the external ther-
mal noise but increases the resulting NF.
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