LT1228CS8 datasheet(13/20 Pages) LINER

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LT1228

APPLICATI

I FOR ATIO

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

Ω

to 20

Ω) 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/

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

VS = ±15V, RF = RG = 750Ω Slew Rate Enhanced

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

feedback 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

VS = ±15V, RF = 1k, RG = 110Ω, RL = 400Ω

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

µs. 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

µV/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.

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