THS3122 Datasheet, PDF (19/43 Pages) Texas Instruments – LOW-NOISE, HIGH-SPEED, 450 mA CURRENT FEEDBACK AMPLIFIERS

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THS3122, THS3125
SLOS382E – SEPTEMBER 2001 – REVISED MAY 2015
Application Information (continued)
Placing a small series resistor, R_ISO, between the amplifier output and the capacitive load, as shown in Figure 42,
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 42. Resistor To Isolate Capacitive Load
Using a ferrite chip in place of R_ISO, as Figure 43 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
Figure 43. Ferrite Bead To Isolate Capacitive Load
Figure 44 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 45 gives
similar results with reduced dc offset and low frequency noise.
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