LT1360 datasheet(12/16 Pages) LINER

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LT1360

Circuit Operation

The LT1360 circuit topology is a true voltage feedback

amplifier that has the slewing behavior of a current feed-

back amplifier. The operation of the circuit can be under-

stood by referring to the simplified schematic. The inputs

are buffered by complementary NPN and PNP emitter

followers which drive a 500

Ω resistor. The input voltage

appears across the resistor generating currents which are

mirrored into the high impedance node. Complementary

followers form an output stage which buffers the gain

node from the load. The bandwidth is set by the input

resistor and the capacitance on the high impedance node.

The slew rate is determined by the current available to

charge the gain node capacitance. This current is the

differential input voltage divided by R1, so the slew rate is

proportional to the input. Highest slew rates are therefore

seen in the lowest gain configurations. For example, a 10V

output step in a gain of 10 has only a 1V input step,

whereas the same output step in unity gain has a 10 times

greater input step. The curve of Slew Rate vs Input Level

illustrates this relationship. The LT1360 is tested for slew

rate in a gain of –2 so higher slew rates can be expected

in gains of 1 and –1, and lower slew rates in higher gain

configurations.

The RC network across the output stage is bootstrapped

when the amplifier is driving a light or moderate load and

has no effect under normal operation. When driving a

capacitive load (or a low value resistive load) the network

is incompletely bootstrapped and adds to the compensa-

tion at the high impedance node. The added capacitance

slows down the amplifier which improves the phase

margin by moving the unity-gain frequency away from the

pole formed by the output impedance and the capacitive

load. The zero created by the RC combination adds phase

to ensure that even for very large load capacitances, the

total phase lag can never exceed 180 degrees (zero phase

margin) and the amplifier remains stable.

Comparison to Current Feedback Amplifiers

The LT1360 enjoys the high slew rates of Current Feed-

back Amplifiers (CFAs) while maintaining the characteris-

tics of a true voltage feedback amplifier. The primary

differences are that the LT1360 has two high impedance

inputs and its closed loop bandwidth decreases as the gain

increases. CFAs have a low impedance inverting input and

maintain relatively constant bandwidth with increasing

gain. The LT1360 can be used in all traditional op amp

configurations including integrators and applications such

as photodiode amplifiers and I-to-V converters where

there may be significant capacitance on the inverting

input. The frequency compensation is internal and not

dependent on the value of the feedback resistor. For CFAs,

the feedback resistance is fixed for a given bandwidth and

capacitance on the inverting input can cause peaking or

oscillations. The slew rate of the LT1360 in noninverting

gain configurations is also superior in most cases.

APPLICATIONS INFORMATION

LT1360 Datasheet(PDF) 12 Page - Linear Technology

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