Datasheet

49.9 W
1 Fm
100- LoadW
+V
S
+V
S
R
G
-V
S
R
F
R
5.11
ISO
W
49.9 W
1 Fm
100- LoadW
+V
S
+V
S
R
G
-V
S
R
F
5.11 W
27pF
560 W
R
IN
49.9 W
1 Fm
100- LoadW
+V
S
+V
S
R
G
-V
S
R
F
Ferrite
Bead
49.9 W
1 Fm
100- LoadW
+V
S
+V
S
R
G
-V
S
R
F
5.11 W
27pF
Ferrite
Bead
F
IN
THS3122
THS3125
www.ti.com SLOS382D –SEPTEMBER 2001– REVISED FEBRUARY 2011
Placing a small series resistor, R
ISO
, between the Figure 44 shows another method used to maintain
amplifier output and the capacitive load, as shown in the low-frequency load independence of the amplifier
Figure 42, is an easy way of isolating the load while isolating the phase shift caused by the
capacitance. 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.
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
Figure 44. Feedback Technique with Input
impedance at dc.
Resistor for Capacitive Load
Figure 43. Ferrite Bead to Isolate Capacitive Load
Figure 45. Feedback Technique with Input Ferrite
Bead for Capacitive Load
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