THS3092_14 Datasheet, PDF (19/45 Pages) Texas Instruments – HIGH-VOLTAGE, LOW-DISTORTION, CURRENT-FEEDBACK OPERATIONAL AMPLIFIERS

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45
Gain = 5,
40
V_S = ±15 V
35
30
25
20
15
715 Ω
10
178 Ω
−
5
+
RISO
CL
0
10
100
C_L − Capacitive Load − pF
Figure 58. Recommended R_ISO vs Capacitive Load
178 Ω
V_S
715 Ω
V_S
_
+
−V_S
49.9 Ω
5.11 Ω ^{100 Ω LOAD}
R_ISO
1 µF
Figure 59.
178 Ω
V_S
715 Ω
V_S
_
+
−V_S
49.9 Ω
Ferrite Bead
100 Ω LOAD
1 µF
Figure 60.
Placing a small series resistor, R_ISO, between the
amplifier’s output and the capacitive load, as shown
in Figure 59, is an easy way of isolating the load
capacitance.
Using a ferrite chip in place of R_ISO, as shown in
Figure 60, is another approach of isolating the output
of the amplifier. The ferrite's 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 - 50 Ω, at 100 MHz and low impedance
at dc.
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.)
R_F
715 Ω
27 pF
R_IN
R_G
178 Ω
V_S
V_S
715 Ω _{_}
+
−V_S
49.9 Ω
5.11 Ω
100 Ω LOAD
1 µF
Figure 61.
F_IN
R_G
178 Ω
V_S
R_F
715 Ω
27 pF
FB ^VS
_
+
−V_S
49.9 Ω
5.11 Ω
100 Ω LOAD
1 µF
Figure 62.
Figure 63 is shown using two amplifiers in parallel to
double the output drive current to larger capacitive
loads. This technique is used when more output
current is needed to charge and discharge the load
faster like when driving large FET transistors.
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