THS3115CPWP Datasheet, PDF (13/35 Pages) Texas Instruments – LOW-NOISE, HIGH-SPEED, CURRENT FEEDBACK AMPLIFIERS

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Placing a small series resistor, R_ISO, between the
amplifier output and the capacitive load, as shown in
Figure 44, is an easy way of isolating the load
capacitance.
R_F
+V_S
R_G
R_ISO
5.11 W
100-W Load
-V_S
+V_S
49.9 W
1 mF
Figure 44. Resistor to Isolate Capacitive Load
Using a ferrite chip in place of R_ISO, as Figure 45
shows, 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 with similar impedance
to R_ISO, 20 Ω to 50 Ω, at 100 MHz and low
impedance at dc.
R_F
+V_S
R_G
Ferrite
Bead
100-W Load
-V_S
+V_S
49.9 W
1 mF
THS3112
THS3115
SLOS385C – SEPTEMBER 2001 – REVISED SEPTEMBER 2010
Figure 46 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 47 gives similar
results with reduced dc offset and low frequency
noise.
R_F
27 pF
+V_S
R_G
750 W
R_IN
-V_S
+V_S
49.9 W
5.11 W ^{100-W Load}
1 mF
Figure 46. Feedback Technique with Input
Resistor for Capacitive Load
R_F
27 pF
+V_S
Ferrite
R_G
Bead
F_IN
-V_S
+V_S
49.9 W
5.11 W ^{100-W Load}
1 mF
Figure 45. Ferrite Bead to Isolate Capacitive Load
Figure 47. Feedback Technique with Input Ferrite
Bead for Capacitive Load
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