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LT1228

1228fd

APPLICATIONS INFORMATION

Capacitive Loads

TheLT1228currentfeedbackamplifiercandrivecapacitive

loadsdirectlywhenthepropervalueoffeedbackresistoris

used.ThegraphofMaximumCapacitiveLoadvsFeedback

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

amplifierismorestableathighergains,anddrivingheavier

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.

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

signalswillturnonthenormallyreversebiasedemitterson

theinputtransistorsandenhancetheslewratesignificantly.

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 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

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 20µs. The thermal

settling component comes from the output and the input

stage. The output contributes just under 1mV/V of output

changeandtheinputcontributes300µV/Vofinputchange.

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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