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0
5
10
15
20
25
30
35
40
45
10 100
C
L
− Capacitive Load − pF
Recommended
R
ISO
Ω
Gain = 5,
V
S
= ±15 V
−
+
−
178 Ω
715 Ω
R
ISO
C
L
_
+
V
S
−V
S
49.9 Ω
715 Ω
5.11 Ω
1 µF
178 Ω
V
S
100 Ω LOAD
R
ISO
_
+
V
S
−V
S
49.9 Ω
5.11 Ω
1 µF
178 Ω
V
S
27 pF
715 Ω
R
F
R
G
715 Ω
100 Ω LOAD
R
IN
_
+
V
S
−V
S
49.9 Ω
715 Ω
Ferrite Bead
1 µF
178 Ω
V
S
100 Ω LOAD
_
+
V
S
−V
S
49.9 Ω
5.11 Ω
1 µF
178 Ω
V
S
27 pF
715 Ω
R
F
R
G
100 Ω LOAD
F
IN
FB
THS3092
THS3096
SLOS428B – DECEMBER 2003 – REVISED FEBRUARY 2006
Figure 61 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 62 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.)
Figure 58. Recommended R
ISO
vs Capacitive Load
Figure 59.
Figure 61.
Figure 60.
Placing a small series resistor, R
ISO
, between the
amplifier’s output and the capacitive load, as shown
Figure 62.
in Figure 59 , is an easy way of isolating the load
capacitance.
Figure 63 is shown using two amplifiers in parallel to
double the output drive current to larger capacitive
Using a ferrite chip in place of R
ISO
, as shown in
loads. This technique is used when more output
Figure 60 , is another approach of isolating the output
current is needed to charge and discharge the load
of the amplifier. The ferrite's impedance characteristic
faster like when driving large FET transistors.
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 - 50 Ω, at 100 MHz and low impedance
at dc.
19
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