12

LT1813/LT1814

18134fa

APPLICATIO S I FOR ATIO

Complementary followers form an output stage that buff-

ers the gain node from the load. The input resistor, input

stage transconductance, and the capacitor on the high

impedance node determine the bandwidth. 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 step. Highest slew rates are therefore seen in the

lowest 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 a heavy load

(capacitive or resistive) is driven, the network is incom-

pletely bootstrapped and adds to the compensation at the

high impedance node. The added capacitance moves the

unity-gain frequency away from the pole formed by the

output impedance and the capacitive load. The zero cre-

ated by the RC combination adds phase to ensure that the

total phase lag does not exceed 180

° (zero phase margin),

and the amplifier remains stable. In this way, the LT1813/

LT1814 are stable with up to 1000pF capacitive loads in

unity gain, and even higher capacitive loads in higher

closed-loop gain configurations.

Power dissipation is composed of two parts. The first is

due to the quiescent supply current and the second is due

to on-chip dissipation caused by the load current. The

worst-case load induced power occurs when the output

voltage is at 1/2 of either supply voltage (or the maximum

is:

Example: LT1814S at 70

Circuit Operation

The LT1813/LT1814 circuit topology is a true voltage

feedback amplifier that has the slewing behavior of a

current feedback amplifier. The operation of the circuit can

be understood by referring to the Simplified Schematic.

Complementary NPN and PNP emitter followers buffer the

inputs and drive an internal resistor. The input voltage

appears across the resistor, generating current that is

mirrored into the high impedance node.

1814 SS

OUT

+IN

–IN

R1

C