Application Hints

The LM321 op amp can operate with a single or dual power

supply voltage, has true-differential inputs, and remain in the

linear mode with an input common-mode voltage of 0 V

This amplifier operates over a wide 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 accommo-

dated and, as input differential voltage protection diodes are

not needed, no large input currents result from large differ-

ential input voltages. The differential input voltage may be

larger than V

+

without damaging the device. Protection

should be provided to prevent the input voltages from going

negative more than −0.3 V

with a resistor to the IC input terminal 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 supply

voltage over the range of from 3 V

Output short circuits either to ground or to the positive power

supply should be of short time duration. Units can be de-

stroyed, 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 exces-

sive 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 general, introduc-

allow operation above and below this 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.

Typical Applications

Non-Inverting DC Gain (0V Input = 0V Output)

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