THS3120 Datasheet, PDF (17/28 Pages) Texas Instruments – LOW-NOISE, HIGH-OUTPUT DRIVE, CURRENT-FEEDBACK, OPERATIONAL AMPLIFIERS

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Driving Capacitive Loads
Applications, such as FET drivers and line drivers can
be highly capacitive and cause stability problems for
high-speed amplifiers.
Figure 49 through Figure 55 show recommended
methods for driving capacitive loads. The basic idea
is to use a resistor or ferrite chip to isolate the phase
shift at high frequency caused by the capacitive load
from the amplifier’s feedback path. See Figure 49 for
recommended resistor values versus capacitive load.
60
Gain = 5,
50
R_L = 50 Ω,
V_S = ±15 V
40
30
20
10
0
10
100
C_L − Capacitive Load − pF
Figure 49. Recommended R_ISO vs Capacitive Load
124 Ω
V_S
499 Ω
V_S
_
+
-V_S
49.9 Ω
5.11 Ω ^{100 Ω LOAD}
R_ISO
1 µF
Figure 50.
124 Ω
V_S
499 Ω
V_S
_
+
-V_S
49.9 Ω
Ferrite Bead
100 Ω LOAD
1 µF
THS3120, THS3121
SLOS420A – SEPTEMBER 2003 – REVISED NOVEMBER 2003
Placing a small series resistor, R_ISO, between the
amplifier’s output and the capacitive load, as shown
in Figure 50, is an easy way of isolating the load
capacitance.
Using a ferrite chip in place of R_ISO, as shown in
Figure 51, is another approach of isolating the output
of the amplifier. The ferrite's impedance characteristic
versus frequency is useful to maintain the low fre-
quency load independence of the amplifier while
isolating the phase shift caused by the capacitance at
high frequency. Use a ferrite with similar impedance
to R_ISO, 20 Ω - 50 Ω, at 100 MHz and low impedance
at dc.
Figure 52 shows another method used to maintain
the low frequency load independence of the amplifier
while isolating the phase shift caused by the capaci-
tance at high frequency. At low frequency, feedback
is mainly from the load side of R_ISO. At high fre-
quency, the feedback is mainly via the 27-pF capaci-
tor. The resistor R_IN in series with the negative input
is used to stabilize the amplifier and should be equal
to the recommended value of R_F at unity gain.
Replacing R_IN with a ferrite of similar impedance at
about 100 MHz as shown in Figure 53 gives similar
results with reduced dc offset and low frequency
noise. (See the ADDITIONAL REFERENCE MA-
TERIAL section for expanding the usability of cur-
rent-feedback amplifiers.)
R_F
499 Ω
27 pF
R_IN
R_G
124 Ω
V_S
V_S
750 Ω _{_}
+
-V_S
49.9 Ω
5.11 Ω
100 Ω LOAD
1 µF
Figure 52.
Figure 51.
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