BL321
Low Power Single Op Amp
APPLICATION SUMMARY
The BL321 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 VDC. 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 VDC (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 BL321 establishes a supply current which is independent of the magnitude of the power
supply voltage over the range of from 3 VDC to 25 VDC
.
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.
Page 5
BL321 Datasheet V1.0
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