SNOS935B – FEBRUARY 2001 – REVISED MARCH 2013
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
DC
. 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 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
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
DC
(at 25°C).An input clamp diode 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
DC
to 30 V
DC
.
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
general, introducing a pseudo-ground (a bias voltage reference of V
+
/2) will 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.
6
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