LM321

SNOS935B – FEBRUARY 2001 – REVISED MARCH 2013

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APPLICATION HINTS

The LM321 op amp can operate with a single or dual power supply voltage, has true-differential inputs, and

range of power supply voltages, with little change in performance characteristics. At 25°C amplifier operation is

possible down to a minimum supply voltage of 3V.

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 differential input voltages. The differential input voltage

from going negative more than

can be used.

To reduce the power supply drain, the amplifier has 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 to reduce distortion.

Capacitive loads which are applied directly to the output of the amplifier reduce the loop stability margin. Values

of 50pF can be accommodated using the worst-case non-inverting unity gain connection. Large closed loop

gains or resistive isolation should be used if large load capacitance must be driven by the amplifier.

The bias network of the LM321 establishes a supply current which is independent of the magnitude of the power

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 junction temperatures. The larger value

of output source current which is available at 25°C provides a larger output current capability at elevated

temperatures than a standard IC op amp.

The circuits presented in the section on typical applications emphasize operation on only a single power supply

voltage. If complementary power supplies are available, all of the standard op amp circuits can be used. In

value in single power supply systems. Many application circuits are shown which take advantage of the wide

input common-mode voltage range which includes ground. In most cases, input biasing is not required and input

voltages which range to ground can easily be accommodated.

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