THS3091 Datasheet, PDF (30/45 Pages) Texas Instruments – HIGH-VOLTAGE, LOW-DISTORTION, CURRENT-FEEDBACK OPERATIONAL AMPLIFIERS

THS3091, THS3095
SLOS423H – SEPTEMBER 2003 – REVISED DECEMBER 2015
Application Information (continued)
249 Ω
V_S
1 kΩ
V_S
_
+
−V_S
49.9 Ω
5.11 Ω ^{100-Ω LOAD}
R_ISO
1 µF
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Figure 65. Driving a Large Capacitive Load Using an Output Series Isolation Resistor
249 Ω
V_S
1 kΩ
V_S
_
+
−V_S
49.9 Ω
Ferrite Bead
1 µF
100-Ω LOAD
Figure 66. Driving a Large Capacitive Load Using an Output Series Ferrite Bead
Placing a small series resistor, R_ISO, between the amplifier’s output and the capacitive load, as shown in
Figure 65, is an easy way of isolating the load capacitance.
Using a ferrite chip in place of R_ISO, as shown in Figure 66, 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 67 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 68 gives
similar results with reduced DC offset and low-frequency noise. (See the Related Documentation section for
expanding the usability of current-feedback amplifiers.)
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