THS4011 Datasheet, PDF (14/36 Pages) Texas Instruments – 290-MHz LOW-DISTORTION HIGH-SPEED AMPLIFIERS

THS4011
THS4012
SLOS216E – JUNE 1999 – REVISED APRIL 2010
ǒ Ǔ _NF
+
ȱ
10log^{ȧȧȧȧȧ}1
ȧȡȢ
)
en
ǒ 2
) IN )
4 kTRS
Ȳ
RSǓ²_{ȧȣȤȧȧȧȧȧȳ}
È´
Figure 26 shows the NF graph for the THS401x.
NOISE FIGURE
vs
SOURCE RESISTANCE
30
f = 10 kHz
T_A = 25°C
25
www.ti.com
20
15
10
5
0
10
100
1k
10 k
100 k
Source Resistance − Ω
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.
1.3 kΩ
Input
1.3 kΩ
_
THS401x
+
20 Ω
Output
C_LOAD
Figure 27. Driving a Capacitive Load
14
Submit Documentation Feedback
Copyright © 1999–2010, Texas Instruments Incorporated
Product Folder Link(s): THS4011 THS4012