LT1228 Linear Technology, LT1228 Datasheet - Page 13

• Manufacturer Part Number: LT1228
• Description: 100MHz Current Feedback Amplifier with DC Gain Control
• Manufacturer: Linear Technology

A
Capacitive Loads
The LT1228 current feedback amplifier can drive capaci-
tive 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 appro-
priate 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 (10
to 20 ) 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.
Slew Rate
The slew rate of the current feedback amplifier is not
independent 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/ s 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/ s!
PPLICATI
V
Current Feedback Amp Large-Signal Response
S
= 15V, R
O
F
U
= R
S
G
= 750 Slew Rate Enhanced
I FOR ATIO
U
W
U
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/ s and –850V/ s. There is no input
stage enhancement because of the high gain. Larger
feedback resistors will reduce the slew rate as will lower
supply voltages, similar to the way the bandwidth is
reduced.
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 20 s. 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 300 V/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.
Current Feedback Amp Large-Signal Response
V
S
= 15V, R
F
= 1k, R
G
= 110 , R
L
= 400
LT1228
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