7

LT1259/LT1260

Feedback Resistor Selection

The small-signal bandwidth of the LT1259/ LT1260 are set

by the external feedback resistors and the internal junction

capacitors. As a result, the bandwidth is a function of the

supply voltage, the value of the feedback resistor, the

closed-loop gain and the load resistor. The LT1259/LT1260

have been optimized for

±5V supply operation and have a

– 3dB bandwidth of 90MHz. See resistor selection guide in

Typical AC Performance table.

Capacitance on the Inverting Input

Current feedback amplifiers require resistive feedback

from the output to the inverting input for stable operation.

Take care to minimize the stray capacitance between the

output and the inverting input. Capacitance on the invert-

ing input to ground will cause peaking in the frequency

response (and overshoot in the transient response). See

the section on Demo Board Information.

Capacitive Loads

The LT1259/LT1260 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 150Ω load at a

gain of 2. This is a worst case condition. The amplifier is

more stable at higher gains. Alternatively, a small resistor

(10

Ω 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. The disadvantage is

that the gain is a function of the load resistance.

Power Supplies

The LT1259/LT1260 will operate from single or split

supplies from

±2V (4V total) to ±15V (30V total). It is not

necessary to use equal value split supplies, however the

offset voltage and inverting input bias current will change.

The offset voltage changes about 500

µV per volt of

supply mismatch. The inverting bias current can change

as much as 5

µA per volt of supply mismatch though

typically, the change is about 0.1

µA per volt.

Slew Rate

The slew rate of a current feedback amplifier is not

independent of the amplifier gain configuration the way

slew rate is in a traditional op amp. This is because both the

input stage and the output stage have slew rate limitations.

In the inverting mode, and for higher gains in the nonin-

verting mode, the signal amplitude between the input pins

is small and the overall slew rate is that of the output stage.

For gains less than ten in the noninverting mode, the

overall slew rate is limited by the input stage.

EN

+IN

–IN

OUT

LT1259/60 • SS

, each amplifier

SI PLIFIED SCHE ATIC

APPLICATIO S I FOR ATIO