Typical Performance Characteristics (Continued)
Input Current (LM2902 only)
Voltage Gain (LM2902 only)
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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 VDC. 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
Capacitive loads which are applied directly to the output of
the amplifier reduce the loop stability margin. Values of 50
pF can be accomodated 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.
to a minimum supply voltage of 2.3 VDC
.
The bias network of the LM158 establishes a drain current
which is independent of the magnitude of the power supply
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
internal conductors and result in a destroyed unit.
voltage over the range of 3 VDC to 30 VDC
.
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 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 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.
Large differential input voltages can be easily accomodated
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 VDC (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 transis-
tors 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.
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.
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.
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