LM7321MA

The LM7321MA is a single-channel high-output-current rail-to-rail input and output operational amplifier from Texas Instruments (originally National Semiconductor), housed in an 8-pin SOIC package. It is designed for applications requiring a wide supply voltage range, high output drive capability, and the ability to drive unlimited capacitive loads without oscillation. NOTE: This device is OBSOLETE; the direct replacement is LM7321MAX/NOPB (same silicon, tape and reel packaging).

The LM7321MA operates over a supply voltage range of 2.5V to 32V (single supply) or +/-1.25V to +/-16V (dual supply), making it suitable for a wide variety of industrial, automotive, and communications applications. The gain bandwidth product is 20 MHz with a slew rate of 18 V/us, providing fast settling for dynamic signals.

A standout feature is the unlimited capacitive load drive capability. Unlike most op amps that oscillate with even moderate capacitive loads (100 pF to 1 nF), the LM7321MA remains stable driving any value of capacitance directly at its output without requiring a series isolation resistor. This makes it ideal for driving long cables, heavy filter capacitances, MOSFET gates, and other highly capacitive loads.

The output can source +65 mA and sink -100 mA (typical), providing strong drive for low-impedance loads. The input common-mode voltage range extends 0.3V beyond both supply rails, allowing true rail-to-rail input operation even with signals that slightly exceed the supplies. The output swings to within approximately 60 mV of the negative rail and 150 mV of the positive rail under typical conditions.

Key specifications at +/-5V: GBW 20 MHz, slew rate 18 V/us, input offset voltage 5 mV max, offset drift +/-2 uV/C, input bias current 0.45 uA typical, supply current 1.1 mA/channel, input voltage noise 15 nV/sqrt(Hz) at 1 kHz, CMRR 80 dB typical, PSRR 78 dB min, THD+N -86 dB.

The device is AEC-Q100 Grade 1 qualified, making it suitable for automotive applications. The operating temperature range is -40C to +125C. The MA suffix denotes SOIC-8 package in tube packaging (95 units per tube). The LM7321MAX/NOPB variant uses tape and reel packaging (2,500 units per reel) and is the recommended replacement.

The device architecture is bipolar, which explains the relatively high input bias current (0.45 uA typical) compared to CMOS op amps, but also provides the superior output drive capability, lower noise, and wider bandwidth. The bipolar input stage contributes to the low 15 nV/sqrt(Hz) voltage noise density, making the LM7321MA suitable for low-noise signal chain applications.

Applications include high-side and low-side current sensing, line drivers, capacitive load drivers, ADC input buffers, and automotive sensor interfaces.

The LM7321MA operates as a single-channel bipolar operational amplifier with rail-to-rail input and output stages, designed for high output drive and unlimited capacitive load stability.

Bipolar Input Stage: Unlike CMOS op amps that use MOSFET differential pairs, the LM7321MA uses a bipolar input stage. This provides lower input voltage noise (15 nV/sqrt(Hz)) and wider bandwidth (20 MHz GBW) but at the cost of higher input bias current (0.45 uA typical) compared to CMOS alternatives. The input stage uses a complementary NPN/PNP pair configuration to achieve rail-to-rail input common-mode range. The NPN pair handles input voltages near the positive rail, while the PNP pair handles voltages near the negative rail. There is a small crossover region in the middle of the common-mode range where both pairs are active, which can cause a slight change in offset voltage and CMRR. The input common-mode range extends 0.3V beyond both supply rails, allowing the amplifier to process signals that slightly exceed the power supplies — useful for overvoltage-protected current sensing circuits.

Rail-to-Rail Output Stage: The output stage uses a complementary emitter-follower configuration capable of sourcing +65 mA and sinking -100 mA. The output voltage swing extends to within approximately 60 mV of the negative rail and 150 mV of the positive rail at light loads. The asymmetric source/sink capability (65 mA source vs 100 mA sink) results from the different saturation voltages of the NPN pull-up and PNP pull-down transistors. This strong output drive enables the amplifier to directly drive low-impedance loads such as 50-ohm cables, LED arrays, and small motors.

Unlimited Capacitive Load Stability: The key differentiating feature of the LM7321MA is its ability to drive any value of capacitive load without oscillation. This is achieved through a special internal compensation scheme that includes a dominant pole and zero network that adapts to the load conditions. In conventional op amps, the capacitive load creates an additional pole at the output node that degrades phase margin and causes oscillation. The LM7321MA compensation network effectively adds a zero that cancels this load-induced pole, maintaining adequate phase margin regardless of the capacitive load value. The trade-off is slightly reduced small-signal bandwidth and increased supply current compared to a conventional uncompensated op amp.

Frequency Compensation: The device is internally compensated for unity-gain stability. The dominant pole is set at approximately 200 Hz, giving a 20 MHz gain-bandwidth product with a -20 dB/decade rolloff to the 0 dB crossing. The phase margin at unity gain is typically 60 degrees without capacitive load and remains adequate (above 30 degrees) with any capacitive load. The slew rate of 18 V/us is set by the available charging current for the internal compensation capacitor.

Power Supply Operation: The wide 2.5V to 32V supply range is achieved through the bipolar architecture, which can tolerate higher voltage swings than CMOS processes. The device operates equally well at 2.7V single supply for low-voltage logic systems and at +/-15V for industrial signal conditioning. The supply current per channel is 1.1 mA typical, which is relatively high for a single op amp but justified by the high output drive and wide bandwidth. The device does not have a shutdown or enable pin, so the supply current is always drawn when powered.