THS3095_14 Datasheet, PDF (19/37 Pages) Texas Instruments – HIGH-VOLTAGE, LOW-DISTORTION, CURRENT-FEEDBACK OPERATIONAL AMPLIFIERS

THS3091
THS3095
www.ti.com........................................................................................................................................ SLOS423G – SEPTEMBER 2003 – REVISED OCTOBER 2008
Driving Capacitive Loads
Applications such as FET line drivers can be highly
capacitive and cause stability problems for
high-speed amplifiers.
Figure 63 through Figure 68 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 63 for
recommended resistor values versus capacitive load.
45
40
35
30
25
20
15
10
5
0
10
Gain = 5,
R_L = 100 Ω,
V_S = ±15 V
100
C_L − Capacitive Load − pF
Figure 63. Recommended R_ISO vs Capacitive Load
249 Ω
V_S
1 kΩ
V_S
_
+
−V_S
49.9 Ω
5.11 Ω ^{100-Ω LOAD}
R_ISO
1 µF
Figure 64.
Placing a small series resistor, R_ISO, between the
amplifier’s output and the capacitive load, as shown
in Figure 64, is an easy way of isolating the load
capacitance.
Using a ferrite chip in place of R_ISO, as shown in
Figure 65, is another approach of isolating the output
of the amplifier. The ferrite's 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 with
similar impedance to R_ISO, 20 Ω to 50 Ω, at 100 MHz
and low impedance at dc.
Figure 66 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 of similar impedance at
about 100 MHz as shown in Figure 67 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.)
R_F
1 kΩ
27 pF
R_IN
R_G
249 Ω
V_S
V_S
1 kΩ _{_}
+
−V_S
49.9 Ω
5.11 Ω
100-Ω LOAD
1 µF
249 Ω
V_S
1 kΩ
V_S
_
+
−V_S
49.9 Ω
Ferrite Bead
1 µF
100-Ω LOAD
Figure 66.
Figure 65.
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