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Many electronic systems need to strengthen, filter, or control small analog signals without using much power or PCB space. The LM321 from Texas Instruments is a single operational amplifier designed for these tasks. This article explains the LM321 specifications, pin configuration, internal operation, input and output limits, typical circuit configurations, performance characteristics, and more.
The LM321 is a low-power, single operational amplifier designed for general analog signal processing. It combines useful performance with low energy consumption, making it suitable for circuits that require efficient operation and a compact design. The device provides a unity-gain bandwidth of about 1 MHz, a specified slew rate of 0.4 V/µs, and a typical quiescent current of approximately 430 µA at a 5 V supply.
Its input common-mode voltage range includes ground, allowing reliable operation from a single power supply as well as from dual supplies. The LM321 can also drive relatively large capacitive loads while maintaining stable performance. It is commonly supplied in a compact 5-pin SOT-23 package, which helps reduce PCB space and component cost.
| Orderable Device | Status | Package Type | Pins | Package Qty | Operating Temperature | Device Marking |
| LM321MF/NOPB | Active | SOT-23 | 5 | 1,000 | −40°C to 85°C | A63A |
| LM321MFX/NOPB | Active | SOT-23 | 5 | 3,000 | −40°C to 85°C | A63A |
| Specification | Rating |
| Maximum supply voltage | 32 V |
| Differential input voltage | ±Supply voltage |
| Input voltage range | −0.3 V to 32 V |
| Maximum input current | 50 mA |
| Maximum junction temperature | 150°C |
| Storage temperature range | −65°C to 150°C |
| Lead soldering temperature | 260°C for 10 seconds |
| Operating supply voltage | 3 V to 30 V |
| Operating temperature range | −40°C to 85°C |
| Input offset voltage | 2 mV typical, 7 mV maximum |
| Input offset current | 5 nA typical, 50 nA maximum |
| Input bias current | 45 nA typical, 250 nA maximum |
| Input common-mode voltage range | 0 V to V+ - 1.5 V |
| Large-signal voltage gain | 100 V/mV typical |
| Power-supply rejection ratio | 100 dB typical |
| Common-mode rejection ratio | 85 dB typical |
| Supply current at 5 V | 0.43 mA typical |
| Output source current | 40 mA typical |
| Output sink current | 20 mA typical |
| Slew rate | 0.4 V/µs |
| Gain-bandwidth product | 1 MHz |
| Phase margin | 60° |
| Total harmonic distortion | 0.015% typical |
| Input voltage-noise density | 40 nV/√Hz |
| Pin Number | Pin Name | Function |
| 1 | +IN | Non-inverting input. A positive voltage change at this pin causes the output voltage to increase. |
| 2 | V− | Negative power-supply connection. It is normally connected to ground in a single-supply circuit. |
| 3 | −IN | Inverting input. A positive voltage change at this pin causes the output voltage to decrease. |
| 4 | OUTPUT | Provides the amplified output signal. |
| 5 | V+ | Positive power-supply connection. |
The LM321 is a single operational amplifier that increases the voltage difference between its non-inverting input (+IN) and inverting input (−IN). Its open-loop gain is very high, so most circuits use negative feedback to control the gain and keep the output stable.
The image is a simplified internal schematic of the LM321 rather than a basic functional block diagram. It shows the internal transistors, current sources, compensation capacitor, and output stage.
The internal circuit can be divided into three main stages:
• Input stage: Transistors Q1 to Q4 compare the voltages applied to +IN and −IN. The PNP input design allows the LM321 to accept input voltages close to ground when using a single power supply.
• Voltage-gain stage: The small difference detected by the input stage is converted into a larger voltage signal. The internal compensation capacitor CC controls the frequency response and helps prevent oscillation when negative feedback is used.
• Output stage: Transistors Q5 to Q13 provide the current needed to drive the output. This stage can both source and sink current, but its output cannot reach the positive supply rail completely. The resistor RSC helps limit the output current during an overload or short circuit.
The LM321 continuously compares the voltage at +IN with the voltage at −IN:
VOUT=AOL (V(+IN)-V(-IN) )
where AOL is the open-loop voltage gain.
When the voltage at +IN is higher than the voltage at −IN, the output moves toward the positive supply. When the voltage at −IN is higher, the output moves toward the negative supply or ground. Because the LM321 has very high open-loop gain, even a small input difference can drive the output close to one of its limits.
In practical circuits, part of the output is normally returned to the inverting input through a negative-feedback network. This feedback keeps the two input voltages nearly equal and allows external resistors to set a stable voltage gain. Depending on the circuit, the LM321 can amplify, buffer, filter, compare, or condition an analog signal.
The LM321 operates from a 3 V to 30 V single supply. It can also use dual supplies, such as ±1.5 V to ±15 V, as long as the total voltage between the two supply pins remains within the recommended range.
Its normal input common-mode range extends from the negative supply rail to approximately 1.5 V below the positive supply at 25°C:
V-≤VIN≤V+-1.5V
For example, with a 5 V supply, the recommended input range is approximately 0 V to 3.5 V. The input can therefore sense signals at ground, but it should not be expected to operate correctly near the positive supply rail.
The output is also not rail-to-rail. It can move very close to the negative supply or ground under a light load, but it normally remains about 1.5 V or more below the positive supply. The exact output range depends on the supply voltage, load resistance, output current, and temperature. For accurate operation, both the input signal and required output voltage must remain within these practical limits.
The input signal is applied to the non-inverting input of the LM321. Resistors R1 and R2 provide negative feedback and set the voltage gain:
Therefore, the output is approximately 101 times the input voltage. A 0 V input ideally produces a 0 V output, while a positive input produces an amplified positive output.
This circuit combines four DC input voltages into one output. Inputs V1 and V2 are applied to the non-inverting input, while V3 and V4 are applied to the inverting input. Because all resistors have the same 100 kΩ value, the output is:
VO=V1+V2-V3-V4
The LM321 increases the output when V1 or V2 rises and decreases it when V3 or V4 rises. All input and output voltages must remain positive in the shown single-supply arrangement.
This circuit uses the LM321 and a 2N7002 MOSFET to control the polarity of the input signal. When the MOSFET is off, the output follows the input. When it is on, the non-inverting input is pulled to ground and the LM321 produces an inverted version of the input. The 200 kHz carrier repeatedly switches between these states, creating an amplitude-modulated output based on the 3 kHz input signal.
The LM321 controls an external transistor to provide more output current than the IC can supply alone. R1 and R2 create negative feedback from the final output, giving a voltage gain of approximately:
The LM321 sets the required output voltage, while the external transistor supplies current to RL. Because feedback is taken after the transistor, the LM321 also compensates for the transistor’s base-emitter voltage drop.
R3 and R4 create a reference voltage for the LM321. Through negative feedback, the IC controls the PNP transistors so that a fixed 2 V appears across R1. With R1=2kΩ, the first output current is:
The second transistor shares the same control voltage and uses R2 to produce another constant current. Equal resistor values and closely matched transistors produce approximately equal currents, I1 and I2.
The LM321 has a typical gain-bandwidth product of 1 MHz. This means its available closed-loop bandwidth decreases as the circuit gain increases. For example, a voltage gain of 10 provides an approximate bandwidth of 100 kHz. The device is suitable for DC and low-frequency signals but is not intended for high-speed communication or wideband video circuits.
The LM321 has a specified slew rate of approximately 0.4 V/µs. Slew rate indicates how quickly the output voltage can change. This speed is sufficient for slowly changing sensor signals, audio-frequency signals, and control voltages. However, large high-frequency signals may become distorted because the output cannot rise or fall fast enough.
Input offset voltage is the small differential voltage required between the inputs to make the output equal to zero. The LM321 has an input offset voltage of a few millivolts, with a maximum value of approximately 7 mV at 25°C. This offset can produce a noticeable output error in high-gain circuits, so the LM321 may not be suitable for very precise measurements of small signals.
The LM321 uses a bipolar input stage and has a typical input bias current of approximately 45 nA, with higher values possible under specified operating conditions. This small current flows through the external input resistors and can create an additional voltage error. Using moderate resistor values and balancing the resistance seen by both inputs can help reduce this error.
The LM321 can typically provide an output current of about 20 mA, but the available current and output-voltage swing depend on the supply voltage and connected load. Its output can approach the negative supply more closely than the positive supply. The device can also handle relatively large capacitive loads, but it is not intended to directly power motors, speakers, or other high-current loads. An external transistor or driver should be used when more current is required.
The LM321 typically draws only about 430 µA of quiescent supply current at a 5 V supply. This corresponds to approximately 2.15 mW of power when no significant load current is being delivered:
P=VS×IQ=5V×430µA≈2.15mW
Its low current consumption makes it useful in battery-powered sensors, portable equipment, and always-on monitoring circuits. Actual power consumption increases when the output drives a load.
• Temperature sensor signal conditioning
• Battery-voltage monitoring
• Low-side current sensing
• Portable measurement devices
• Active low-pass and high-pass filters
• Audio signal preamplifiers
• Photodiode signal amplification
• Pressure sensor interfaces
• Data-acquisition systems
• Battery-powered medical devices, etc.
| Parameter | LM321 | LM321LV | LMV321-N | LM358 | TLV9301 |
| Number of channels | 1 | 1 | 1 | 2 | 1 |
| Supply-voltage range | 3–32 V | 2.7–5.5 V | 2.7–5.5 V | 3–30 V | 4.5–40 V |
| Gain-bandwidth product | 1 MHz | 1 MHz | 1 MHz | 0.7 MHz | 1 MHz |
| Typical slew rate | 0.5 V/µs | 0.5 V/µs | 1 V/µs | 0.3 V/µs | 3 V/µs |
| Maximum input offset voltage at 25°C | 7 mV | 3 mV | 7 mV | 7 mV | 2.5 mV |
| Typical supply current per channel | 430–450 µA | 90 µA | 130 µA | 350 µA | 150 µA |
| Input stage | Bipolar | CMOS | Bipolar | Bipolar | CMOS |
| Rail-to-rail output | No | No | Yes | No | Yes |
| Main advantage | Wide supply range in a small package | Low-voltage and low-power operation | Faster low-voltage operation | Two amplifiers in one IC | Faster, more accurate operation |
| Best suited for | General-purpose single-op-amp circuits | Battery-powered low-voltage circuits | Portable low-voltage devices | Dual-channel analog circuits | Improved high-voltage designs |
Texas Instruments (TI) designs and manufactures the LM321 using its established analog semiconductor processes. Its manufacturing capabilities cover chip design, wafer fabrication, wafer probing, packaging, electrical testing, and final product delivery. During production, each LM321 die is tested for functionality and key electrical characteristics before being assembled in its compact five-pin SOT-23 package. Finished devices undergo additional testing to verify parameters such as supply current, offset voltage, gain, input bias current, and output performance. TI applies quality and reliability controls throughout the supply chain, helping the LM321 provide consistent operation in consumer, industrial, and battery-powered circuits.
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