Diodes AP358 - Manual (Page 13)

AP358
LOW POWER DUAL OPERATIONAL AMPLIFIERS
AP358
Document number: DS31007 Rev. 6 - 2
13 of 16
www.diodes.com
July 2010
© Diodes Incorporated
Application Information
The AP358 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 50pF 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 AP358 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 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, 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.