LMH6654 Datasheet, PDF (19/31 Pages) National Semiconductor (TI) – Single/Dual Low Power, 250 MHz, Low Noise Amplifiers

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LMH6654, LMH6655
SNOS956E – JUNE 2001 – REVISED AUGUST 2014
Typical Application (continued)
7.2.2.3.1 Noise Figure
Noise Figure (NF) is a measure of the noise degradation caused by an amplifier.
S_i/N_i
e_ni²
NF = 10LOG
= 10LOG
S_o/N_o
e_t²
(5)
The noise figure formula is shown in Equation 5. The addition of a terminating resistor R_T, reduces the external
thermal noise but increases the resulting NF.
The NF is increased because the R_T reduces the input signal amplitude thus reducing the input SNR.
e_n2 + i_n2 (R_Seq + (R_f || R_g))² + 4KTR_Seq + 4kt (R_f || R_g)
4kTR_Seq
(6)
The noise figure is related to the equivalent source resistance (R_seq) and the parallel combination of R_f and R_g.
To minimize noise figure, the following steps are recommended:
1. Minimize R_f||R_g
2. Choose the Optimum R_s (R_OPT)
R_OPT is the point at which the NF curve reaches a minimum and is approximated by:
R_OPT ≈ (e_n/i_n)
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