Datasheet
Driving Capacitive Loads
0
5
10
15
20
25
30
35
40
45
10 100
C
L
− Capacitive Load − pF
Recommended
R
ISO
Ω
Gain = 5,
R
L
= 100 Ω,
V
S
= ±15 V
−
_
+
V
S
−V
S
49.9 Ω
1 kΩ
5.11 Ω
1 µF
249 Ω
V
S
100-Ω LOAD
R
ISO
_
+
V
S
−V
S
49.9 Ω
5.11 Ω
1 µF
249 Ω
V
S
27 pF
1 kΩ
R
F
R
G
1 kΩ
100-Ω LOAD
R
IN
_
+
V
S
−V
S
49.9 Ω
1 kΩ
Ferrite Bead
1 µF
249 Ω
V
S
100-Ω LOAD
THS3091
THS3095
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........................................................................................................................................ SLOS423G – SEPTEMBER 2003 – REVISED OCTOBER 2008
Placing a small series resistor, R
ISO
, between the
amplifier ’ s output and the capacitive load, as shown
Applications such as FET line drivers can be highly
in Figure 64 , is an easy way of isolating the load
capacitive and cause stability problems for
capacitance.
high-speed amplifiers.
Using a ferrite chip in place of R
ISO
, as shown in
Figure 63 through Figure 68 show recommended
Figure 65 , is another approach of isolating the output
methods for driving capacitive loads. The basic idea
of the amplifier. The ferrite's impedance characteristic
is to use a resistor or ferrite chip to isolate the phase
versus frequency is useful to maintain the
shift at high frequency caused by the capacitive load
low-frequency load independence of the amplifier
from the amplifier ’ s feedback path. See Figure 63 for
while isolating the phase shift caused by the
recommended resistor values versus capacitive load.
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 66 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 67 gives similar
results with reduced dc offset and low-frequency
Figure 63. Recommended R
ISO
vs Capacitive Load
noise. (See the ADDITIONAL REFERENCE
MATERIAL section for expanding the usability of
current-feedback amplifiers.)
Figure 64.
Figure 66.
Figure 65.
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