THS3110IDGNR Datasheet, PDF (18/37 Pages) Texas Instruments – LOW-NOISE, HIGH-VOLTAGE, CURRENT-FEEDBACK

THS3110
THS3111
SLOS422E – SEPTEMBER 2003 – REVISED OCTOBER 2009........................................................................................................................................ www.ti.com
Driving Capacitive Loads
Applications such as FET drivers and line drivers can
be highly capacitive and cause stability problems for
high-speed amplifiers.
Figure 55 through Figure 61 show recommended
methods for driving capacitive loads. The basic idea
is to use a resistor or ferrite chip to isolate the phase
shift at high frequency caused by the capacitive load
from the amplifier feedback path. See Figure 55 for
recommended resistor values versus capacitive load.
60
Gain = 5,
50
R_L = 100 Ω,
V_S = ±15 V
40
frequency load independence of the amplifier while
isolating the phase shift caused by the capacitance at
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.
200 Ω
V_S
806 Ω
V_S
_
+
-V_S
49.9 Ω
Ferrite Bead
1 µF
100 Ω LOAD
30
20
10
0
10
100
C_L - Capacitive Load - pF
Figure 55. Recommended R_ISO vs Capacitive
Load
Placing a small series resistor, R_ISO, between the
amplifier output and the capacitive load, as shown in
Figure 56, is an easy way of isolating the load
capacitance.
200 Ω
V_S
806 Ω
V_S
_
+
-V_S
49.9 Ω
5.11 Ω ^{100 Ω LOAD}
R_ISO
1 µF
Figure 56. Resistor to Isolate Capacitive Load
Figure 57. Ferrite Bead to Isolate Capacitive Load
Figure 58 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 59 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.)
R_F
806 Ω
27 pF
R_IN
R_G
200 Ω
V_S
V_S
750 Ω _{_}
+
-V_S
49.9 Ω
5.11 Ω
100 Ω LOAD
1 µF
Using a ferrite chip in place of R_ISO, as shown in
Figure 57, is another approach of isolating the output
of the amplifier. The ferrite impedance characteristic
versus frequency is useful to maintain the low
Figure 58. Feedback Technique with Input
Resistor for Capacitive Load
18
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