LT1228_12 Datasheet, PDF (13/22 Pages) Linear Technology – 100MHz Current Feedback

LT1228
APPLICATIONS INFORMATION
Capacitive Loads
The LT1228 current feedback amplifier can drive capacitive
loads directly when the proper value of feedback resistor is
used. The graph of Maximum Capacitive Load vs Feedback
Resistor should be used to select the appropriate value.
The value shown is for 5dB peaking when driving a 1k
load, at a gain of 2. This is a worst case condition, the
amplifier is more stable at higher gains, and driving heavier
loads. Alternatively, a small resistor (10Ω to 20Ω) can be
put in series with the output to isolate the capacitive load
from the amplifier output. This has the advantage that the
amplifier bandwidth is only reduced when the capacitive
load is present and the disadvantage that the gain is a
function of the load resistance.
The output slew rate is set by the value of the feedback
resistors and the internal capacitance. At a gain of ten with
a 1k feedback resistor and ±15V supplies, the output slew
rate is typically 500V/µs and –850V/µs. There is no input
stage enhancement because of the high gain. Larger feed-
back resistors will reduce the slew rate as will lower supply
voltages, similar to the way the bandwidth is reduced.
Current Feedback Amp Large-Signal Response
V_S = ±15V, R_F = 1k, R_G = 110Ω, R_L = 400Ω
Slew Rate
The slew rate of the current feedback amplifier is not inde-
pendent of the amplifier gain configuration the way it is in
a traditional op amp. This is because the input stage and
the output stage both have slew rate limitations. The input
stage of the LT1228 current feedback amplifier slews at
about 100V/µs before it becomes nonlinear. Faster input
signals will turn on the normally reverse biased emitters on
the input transistors and enhance the slew rate significantly.
This enhanced slew rate can be as much as 3500V/µs!
Current Feedback Amp Large-Signal Response
V_S = ±15V, R_F = R_G = 750Ω Slew Rate Enhanced
Settling Time
The characteristic curves show that the LT1228 current
feedback amplifier settles to within 10mV of final value
in 40ns to 55ns for any output step less than 10V. The
curve of settling to 1mV of final value shows that there
is a slower thermal contribution up to 20µs. The thermal
settling component comes from the output and the input
stage. The output contributes just under 1mV/V of output
change and the input contributes 300µV/V of input change.
Fortunately the input thermal tends to cancel the output
thermal. For this reason the noninverting gain of two
configuration settles faster than the inverting gain of one.
1228fd
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