APPLICATION HINTS

The

LM2904 is op amps which operate with only a single power supply voltage, have

true-differential inputs, and remain in the linear mode with an input common-mode voltage

of 0V. These amplifiers operate over a wide range of power supply voltage with little

change in performance characteristics. At 25°C amplifier operation is possible down to a

minimum supply voltage of 2.3 V.

Precautions should be taken to insure that the power supply for the integrated circuit

never becomes reversed in polarity or that the unit is not inadvertently installed backwards

in a test socket as an unlimited current surge through the resulting forward diode within the

IC could cause fusing of the internal conductors and result in a destroyed unit.

Large differential input voltages can be easily accommodated and, as input differential

voltage protection diodes are not needed, no large input currents result from large

damaging the device. Protection should be provided to prevent the input voltages from

going negative more than -0.3V (at 25°C). An input clamp diode with a resistor to the IC

input terminal can be used.

To reduce the power supply current drain, the amplifiers have a class A output stage for

small signal levels which converts to class B in a large signal mode. This allows the

amplifiers to both source and sink large output currents. Therefore both NPN and PNP

external current boost transistors can be used to extend the power capability of the basic

amplifiers. The output voltage needs to raise approximately 1 diode drop above ground to

bias the on-chip vertical PNP transistor for output current sinking applications.

For ac applications, where the load is capacitively coupled to the output of the amplifier,

a resistor should be used, from the output of the amplifier to ground to increase the class A

bias current and prevent crossover distortion. Where the load is directly coupled, as in dc

applications, there is no crossover distortion.

Capacitive loads which are applied directly to the output of the amplifier reduce the loop

stability margin. Values of 50pF can be accomodated using the worst-case non-inverting

unity gain connection. Large closed loop gains or resistive isolation should be used if larger

load capacitance must be driven by the amplifier.

Output short circuits either to ground or to the positive power supply should be of short

time duration. Units can be destroyed, not as a result of the short circuit current causing

metal fusing, but rather due to the large increase in IC chip dissipation which will cause

eventual failure due to excessive function temperatures. Putting direct short-circuits on

more than one amplifier at a time will increase the total IC power dissipation to destructive

levels, if not properly protected with external dissipation limiting resistors in series with

the output leads of the amplifiers. The larger value of output source current which is

available at 25°C provides a larger output current capability at elevated temperatures (see

typical performance characteristics) than a standard IC op amp.

LM2904N/D

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