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LT1497

The LT1497 is a dual current feedback amplifier with high

output current drive capability. Bandwidth is maintained

over a wide range of voltage gains by the appropriate

choice of feedback resistor. These amplifiers will drive low

impedance loads such as cables with excellent linearity at

high frequencies.

Feedback Resistor Selection

The optimum value for the feedback resistor is a function

of the operating conditions of the device, the load imped-

ance and the desired flatness of frequency response. The

Small-Signal Bandwidth table gives the values which

result in the highest bandwidth with less than 1dB of

peaking for various gains, loads and supply voltages. If

this level of flatness is not required, a higher bandwidth

can be obtained by use of a lower feedback resistor. The

characteristic curves of Bandwidth vs Supply Voltage

indicate feedback resistors for peaking up to 5dB. These

curves use a solid line when the response has less than

1dB of peaking and a dashed line when the response has

1dB to 5dB of peaking. Note that in a gain of 10 peaking is

always under 1dB for the resistor ranges shown. Reducing

the feedback resistor further than 270

Ω in a gain of 10 will

increase the bandwidth, but it also loads the amplifier and

reduces the maximum current available to drive the load.

Capacitive Loads

The LT1497 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 graph shows

feedback resistor values for 5dB frequency 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 (smaller load resistors). Alterna-

tively, 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 in that the ampli-

fier bandwidth is only reduced when the capacitive load is

present, and the disadvantage that the gain is a function of

the load resistance.

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), but it

does not degrade the stability of the amplifier.

Power Supplies

The LT1497 will operate on 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 1mV per volt of supply mismatch.

The inverting bias current can change as much as 10

µA

per volt of supply mismatch, though typically the change

is less than 2.5

µA per volt.

Thermal Considerations

The LT1497 contains a thermal shutdown feature that

protects against excessive internal (junction) tempera-

ture. If the junction temperature of the device exceeds the

protection threshold, the device will begin cycling

between normal operation and an off state. The cycling is

not harmful to the part. The thermal cycling occurs at a

slow rate, typically 10ms to several seconds, depending

upon the power dissipation and the thermal time con-

stants of the package and the amount of copper on the

board under the package. Raising the ambient tempera-

ture until the device begins thermal shutdown gives a

good indication of how much margin there is in the

thermal design.

For surface mount devices heat sinking is accomplished

by using the heat spreading capabilities of the PC board

and its copper traces. Experiments have shown that the

heat spreading copper layer does not need to be electri-

cally connected to the leads of the device. The PCB

material can be very effective at transmitting heat between