NF

+ 10log 1 )

e

n

2

) IN )

R

S

2

4 kTR

S

20

5

0

10

100

25

NOISEFIGURE

vs

SOURCERESISTANCE

30

1k

100k

15

10

Source Resistance − Ω

f=10kHz

10k

+

_

THS401x

1.3kΩ

Input

Output

1.3kΩ

20 Ω

THS4011

THS4012

SLOS216E – JUNE 1999 – REVISED APRIL 2010

www.ti.com

Figure 26 shows the NF graph for the THS401x.

Figure 26. Noise Figure vs Source Resistance

DRIVING A CAPACITIVE LOAD

Driving capacitive loads with high performance amplifiers is not a problem, as long as certain precautions are

taken. The first precaution is to note that the THS401x has been internally compensated to maximize its

bandwidth and slew-rate performance. When the amplifier is compensated in this manner, capacitive loading

directly on the output decreases the device phase margin leading to high-frequency ringing or oscillations.

Therefore, for capacitive loads of greater than 10 pF, it is recommended that a resistor be placed in series with

the output of the amplifier, as shown in Figure 27. A minimum value of 20

Ω should work well for most

applications. For example, in 75-

Ω transmission systems, setting the series-resistor value to 75 Ω both isolates

any capacitance loading and provides the proper line-impedance matching at the source end.

Figure 27. Driving a Capacitive Load

14

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