THS3001 Datasheet, PDF (21/32 Pages) Texas Instruments – 420-MHz HIGH-SPEED CURRENT-FEEDBACK AMPLIFIERS

THS3001, THS3002
420-MHz HIGH-SPEED CURRENT-FEEDBACK AMPLIFIERS
APPLICATION INFORMATION
SLOS217A – JULY 1998 – REVISED JUNE 1999
noise calculations and noise figure (continued)
As the previous equations show, to keep noise at a minimum, small value resistors should be used. As the
closed-loop gain is increased (by reducing R_G), the input noise is reduced considerably because of the parallel
resistance term. This leads to the general conclusion that the most dominant noise sources are the source
resistor (R_S) and the internal amplifier noise voltage (e_n). Because noise is summed in a root-mean-squares
method, noise sources smaller than 25% of the largest noise source can be effectively ignored. This can greatly
simplify the formula and make noise calculations much easier.
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.
+ _ȧȱȲ ^ȳȴȧ NF
10log
eni ²
eRs2
Because the dominant noise components are generally the source resistance and the internal amplifier noise
voltage, we can approximate noise figure as:
ȧȧȧȧȱȲ ^{ȧȡȢǒ Ǔ ǒ Ǔ ȧȣȤȳȴȧȧȧȧ} _{+ ) NF 10log 1}
) ) ²
en
IN
4 kTRS
2
RS
The Figure 50 shows the noise figure graph for the THS300x.
NOISE FIGURE
vs
SOURCE RESISTANCE
20
f = 10 kHz
18 TA = 25°C
16
14
12
10
8
6
4
2
0
10
100
1k
10k
RS – Source Resistance – Ω
Figure 50. Noise Figure vs Source Resistance
• POST OFFICE BOX 655303 DALLAS, TEXAS 75265
21