THS3120_14 Datasheet, PDF (17/36 Pages) Texas Instruments – LOW-NOISE, HIGH-OUTPUT DRIVE, CURRENT-FEEDBACK OPERATIONAL AMPLIFIERS

THS3120
THS3121
www.ti.com........................................................................................................................................ SLOS420E – SEPTEMBER 2003 – REVISED OCTOBER 2009
Driving Capacitive Loads
Applications such as FET drivers and line drivers can
be highly capacitive and cause stability problems for
high-speed amplifiers.
Figure 51 through Figure 57 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 feedback path. See Figure 51 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 51. Recommended R_ISO vs Capacitive
Load
Placing a small series resistor, R_ISO, between the
amplifier output and the capacitive load, as shown in
Figure 52, is an easy way of isolating the load
capacitance.
Using a ferrite chip in place of R_ISO, as shown in
Figure 53, is another approach of isolating the output
of the amplifier. The ferrite impedance characteristic
versus frequency is useful to maintain the low
frequency load independence of the amplifier while
isolating the phase shift caused by the capacitance at
high frequency. Use a ferrite chip with similar
impedance to R_ISO, 20 Ω to 50 Ω, at 100 MHz and
low impedance at dc.
Figure 54 shows another method used to maintain
the low frequency load independence of the amplifier
while isolating the phase shift caused by the
capacitance at high frequency. At low frequency,
feedback is mainly from the load side of R_ISO. At high
frequency, the feedback is mainly via the 27-pF
capacitor. 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 chip of similar impedance
at about 100 MHz as illustrated in Figure 55 gives
similar results with reduced dc offset and low
frequency noise. (See the Additional Reference
Material section for expanding the usability of
current-feedback amplifiers.)
124 Ω
V_S
499 Ω
V_S
_
+
-V_S
49.9 Ω
5.11 Ω ^{100 Ω LOAD}
R_ISO
1 µF
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. Resistor to Isolate Capacitive Load
124 Ω
V_S
499 Ω
V_S
_
+
-V_S
49.9 Ω
Ferrite Bead
1 µF
100 Ω LOAD
Figure 54. Feedback Technique with Input
Resistor for Capacitive Load
Figure 53. Ferrite Bead to Isolate Capacitive Load
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