Datasheet

0
10
20
30
40
50
60
10 100
C
L
− Capacitive Load − pF
Recommended R Ω
Gain = 5,
R
L
= 50 Ω,
V
S
= ±15 V
ISO
Resistance −
_
+
V
S
-V
S
49.9 Ω
499 Ω
5.11 Ω
1 µF
124 Ω
V
S
100 Ω LOAD
R
ISO
_
+
V
S
-V
S
49.9 Ω
5.11 Ω
1 µF
124 Ω
V
S
27 pF
499 Ω
R
F
R
G
750 Ω
100 Ω LOAD
R
IN
_
+
V
S
-V
S
49.9 Ω
499 Ω
Ferrite Bead
1 µF
124 Ω
V
S
100 Ω LOAD
THS3120
THS3121
www.ti.com ........................................................................................................................................ SLOS420E –SEPTEMBER 2003–REVISED OCTOBER 2009
Driving Capacitive Loads
Placing a small series resistor, R
ISO
, between the
amplifier output and the capacitive load, as shown in
Applications such as FET drivers and line drivers can
Figure 52, is an easy way of isolating the load
be highly capacitive and cause stability problems for
capacitance.
high-speed amplifiers.
Using a ferrite chip in place of R
ISO
, as shown in
Figure 51 through Figure 57 show recommended
Figure 53, is another approach of isolating the output
methods for driving capacitive loads. The basic idea
of the amplifier. The ferrite impedance characteristic
is to use a resistor or ferrite chip to isolate the phase
versus frequency is useful to maintain the low
shift at high frequency caused by the capacitive load
frequency load independence of the amplifier while
from the amplifier feedback path. See Figure 51 for
isolating the phase shift caused by the capacitance at
recommended resistor values versus capacitive load.
high frequency. Use a ferrite chip with similar
impedance to R
ISO
, 20 Ω to 50 Ω, at 100 MHz and
low impedance at dc.
Figure 54 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 chip of similar impedance
at about 100 MHz as illustrated in Figure 55 gives
similar results with reduced dc offset and low
frequency noise. (See the Additional Reference
Figure 51. Recommended R
ISO
vs Capacitive
Material section for expanding the usability of
Load
current-feedback amplifiers.)
Figure 52. Resistor to Isolate Capacitive Load
Figure 54. Feedback Technique with Input
Resistor for Capacitive Load
Figure 53. Ferrite Bead to Isolate Capacitive Load
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