THS7001 Datasheet, PDF (17/32 Pages) Texas Instruments – 70-MHz PROGRAMMABLE-GAIN AMPLIFIERS

THS7001, THS7002
70-MHz PROGRAMMABLE-GAIN AMPLIFIERS
SLOS214B – OCTOBER 1998 – REVISED AUGUST 1999
APPLICATION INFORMATION
noise calculations and noise figure (continued)
RF1
RF2
RG1
+
Vin
eni1
_
eno1
A1
+
RF1
AV1= 1+ _RG1
RG2
_
+
eni2
RF2
AV2= 1+ _RG2
eno2
Figure 53. Simplified Composite Amplifier System
The noise due to amplifier 1 (A1) is the same as derived in equations 1 and 2. The composite system noise is
calculated as follows:
+ Ǹ ) eno2
eni2² eno1² AV2
+ ^Ǹ ) ^{ǒ Ǔ} eni2²
2
eni1AV1 AV2
(3)
In a typical system, amplifier 1 (A1) has a large gain (A_V1). Because the noise is summed in the RMS method,
if the A1 output noise is more than 25% larger than the input noise of amplifier 2, the contribution of
amplifier 2’s input noise to the composite amplifier output noise can effectively be ignored. This reduces
equation 3 down to:
eno2 ≅ eni1 AV1 AV2
(4)
Equation 4 shows that the very first amplifier (the preamplifier) is critical in any low-level signal system. This also
shows that practically any noisy amplifier can be used after the preamplifier with minimal SNR degradation.
For more information on noise analysis, please refer to the Noise Analysis section in Operational Amplifier
Circuits Applications Report (literature number SLVA043).
This brings up another noise measurement usually preferred in RF applications, the noise figure (NF). Noise
figure is a measure of noise degradation caused by the amplifier. The value of the source resistance must be
defined and is typically 50 Ω in RF applications.
+ _ȧȱ ^ȳȧ NF
10log
eni ²
2
Ȳ^{ǒ Ǔ} ȴ ^eRs
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