THS3001, THS3002

420-MHz HIGH-SPEED CURRENT-FEEDBACK AMPLIFIERS

SLOS217A – JULY 1998 – REVISED JUNE 1999

21

POST OFFICE BOX 655303

APPLICATION INFORMATION

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

resistance term. This leads to the general conclusion that the most dominant noise sources are the source

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.

NF

+ 10log

e

2

ni

e

Rs2

Because the dominant noise components are generally the source resistance and the internal amplifier noise

voltage, we can approximate noise figure as:

NF

+ 10log 1 )

e

n

2

) IN )

R

S

2

4kTR

S

The Figure 50 shows the noise figure graph for the THS300x.

NOISE FIGURE

vs

SOURCE RESISTANCE

10

100

10k

RS – Source Resistance – Ω

1k

f = 10 kHz

TA = 25°C

12

8

4

0

10

6

2

20

16

18

14

Figure 50. Noise Figure vs Source Resistance