Datasheet

10 100 1k 10k 100k
R
S
(:)
Noise Figure (dB)
18
16
14
12
10
8
6
4
2
0
Unterminated
Terminated
f > 1 MHz
G
1
NF = 10 log
¸
¹
·
¨
©
§
¸
¹
·
¨
©
§
G(N
i
+ N
a
)
N
i
= 10 log 1 +
N
i
N
a
LMH6629
SNOSB18G –APRIL 2010–REVISED MARCH 2013 www.ti.com
Looking at the two parts of the NF expression (inside the log function) yields:
S
i
/ S
o
→ Inverse of the power gain provided by the amplifier
N
o
/ N
i
→ Total output noise power, including the contribution of R
S
, divided by the noise power at the input due to
R
S
To simplify this, consider N
a
as the noise power added by the amplifier (reflected to its input port):
S
i
/ S
o
→ 1/ G
N
o
/ N
i
→ G * (N
i
+N
a
) / N
i
(where G*(N
i
+N
a
) = N
o
)
Substituting these two expressions into the NF expression:
Equation 6: Simplified Noise Figure Equation (6)
The noise figure expression has simplified to depend only on the ratio of the noise power added by the amplifier
at its input (considering the source resistor to be in place but noiseless in getting N
a
) to the noise power
delivered by the source resistor (considering all amplifier elements to be in place but noiseless in getting N
i
).
For a given amplifier with a desired closed loop gain, to minimize noise figure:
• Minimize R
f
|| R
g
• 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
(7)
Figure 61 is a plot of NF vs R
S
with the circuit of Figure 55 (R
f
= 240Ω, A
V
= +10V/V). The NF curves for both
Unterminated (R
T
= open) and Terminated systems (R
T
= R
S
) are shown. Table 1 indicates NF for various source
resistances including R
S
= R
OPT
.
Figure 61. Noise Figure vs. Source Resistance
Table 1. Noise Figure for Various R
s
R
S
(Ω) NF (Terminated) (dB) NF (Unterminated) (dB)
50 8 3.2
R
OPT
4.1 1.1
(R
OPT
= 750Ω) (R
OPT
= 350Ω)
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