LM124/LM224/LM324/LM2902
Typical Performance Characteristics
Input Current (LM2902 only)
(Continued)
Voltage Gain (LM2902 only)
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Application Hints
The LM124 series are 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 0 V
DC
. 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
DC
.
The pinouts of the package have been designed to simplify
PC board layouts. Inverting inputs are adjacent to outputs for
all of the amplifiers and the outputs have also been placed at
the corners of the package (pins 1, 7, 8, and 14).
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 result-
ing forward diode within the IC could cause fusing of the in-
ternal conductors and result in a destroyed unit.
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 differen-
tial 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 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
50 pF can be accommodated using the worst-case
non-inverting unity gain connection. Large closed loop gains
or resistive isolation should be used if larger load capaci-
tance must be driven by the amplifier.
The bias network of the LM124 establishes a drain 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 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. 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 pro-
tected 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.
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-
ing 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.
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