SNOSBT3H – JANUARY 2000 – REVISED MARCH 2013
APPLICATION HINTS
The LM158 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
.
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 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 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 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 capacitance must be driven by the amplifier.
The bias network of the LM158 establishes a drain current which is independent of the magnitude of the power
supply voltage over the range of 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 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
than a standard IC
op amp.
The circuits presented in the
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.
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11
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