Small analog signals from sensors and other electronic circuits often need to be amplified, filtered, or buffered before another device can use them. This article explains the LM2904N internal circuit, pinout, specifications, voltage limits, supply configurations, design formulas, application circuits, performance curve, advantages, limitations, replacement options, and common circuit problems. These details will help you decide whether the LM2904N is suitable for your circuit and use it within safe operating limits.

Catalog

LM2904N Dual-Operational Amplifiers

The <a href="https://www.y-ic.com/pdf/Luminary-Micro-Texas-Instruments/LM2904N.html" target="_blank" "="" style="cursor: pointer; color: rgb(0, 0, 238); font-weight: bold;">LM2904N is a general-purpose integrated circuit containing two independent operational amplifiers. It amplifies and processes analog signals using either a single power source or split supplies. Internal frequency compensation also allows stable operation in many negative-feedback circuits.

The LM2904N is commonly used in sensor signal conditioning, voltage amplification, active filters, voltage followers, and other low-power analog circuits. Its simple design makes it suitable for industrial equipment, control systems, prototypes, repairs, and educational projects.

LM2904N Internal Circuit

The diagram shows the internal circuit of one operational-amplifier channel in the LM2904N. The complete IC contains two similar channels that share the same power-supply connections. The input stage begins with transistors Q1 to Q4, which form a differential-input network. This stage compares the voltages at the inverting (−) and non-inverting (+) inputs. Current sources provide controlled bias currents so the transistors operate consistently while consuming little power.

Transistors Q8 and Q9 form part of the active-load and current-mirror network. They convert the differential input into a signal that can be amplified by the following stages. The internal compensation capacitor C_Ccontrols the frequency response and helps prevent unwanted oscillation when negative feedback is used.

The amplified signal then passes through Q10, Q11, Q12, and Q7 to the output stage. Transistors Q5, Q6, and Q13 control the output voltage and provide current to or from the connected load. Resistor R_SCand the associated transistors help limit excessive output current. Together, these internal stages provide high voltage gain, stable operation, low power consumption, and short-circuit protection.

LM2904N Pinout and CAD Model

LM2904N Pinout

CAD Model

LM2904N Features and Specifications

LM2904N Features

• Two independent operational amplifiers

• 8-pin PDIP through-hole package

• Single-supply operation from 3 V to 26 V

• Split-supply operation from ±1.5 V to ±13 V

• Typical unity-gain bandwidth of 1 MHz

• Typical slew rate of 0.24 V/µs

• Maximum input-offset voltage of 7 mV at 25°C

• Typical supply current of 500 µA for both channels

• Input common-mode range includes the negative supply or ground

• Output can swing close to ground under suitable load conditions

• Internally frequency compensated

• Stable at unity gain

• Low input-bias current

• Short-circuit-protected outputs

• Operating-temperature range from −40°C to +85°C

• Supports single- and dual-supply circuit designs

• Bipolar-input architecture

• Not a rail-to-rail input or output op-amp

LM2904N Technical Specifications

LM2904N Input and Output Voltage Limits

The LM2904N is not a fully rail-to-rail operational amplifier. Its input common-mode range includes the negative supply, which is normally ground in a single-supply circuit. This allows the device to measure and amplify signals close to 0 V without requiring a negative power supply. However, the input voltage cannot normally reach the positive supply rail. It must remain approximately 1.5 V or more below the positive supply, depending on temperature and operating conditions.

The output can also move close to ground when the connected load is suitable. However, it cannot reach the positive supply voltage. For example, with a 5 V supply, the highest usable output may be considerably below 5 V. The exact limit depends on the output current, load resistance, temperature, and individual device characteristics.

Heavier loads reduce the available output-voltage swing because the output stage must provide more current. Designers should therefore check both the input common-mode range and output-swing specifications under the expected load. Treating the LM2904N as a rail-to-rail op-amp may cause clipping, inaccurate measurements, or an output that becomes stuck near a supply limit.

LM2904N Single-Supply and Dual-Supply Operation

For single-supply operation, connect pin 8 to the positive supply and pin 4 to ground. The LM2904N typically operates from a total supply voltage of 3 V to 26 V. Because the output cannot move below ground, AC signals that swing in both directions should be biased above ground. A resistor divider can create a midpoint reference, such as 2.5 V in a 5 V system, allowing the signal to move above and below this reference without becoming negative.

For dual-supply operation, connect pin 8 to the positive rail and pin 4 to the negative rail. A symmetrical supply may range from approximately ±1.5 V to ±13 V. System ground is then located between the two rails. This arrangement allows signals and outputs to move above and below ground, provided they remain within the LM2904N’s input and output limits.

Place a 0.1 µF ceramic decoupling capacitor close to pins 8 and 4. A larger capacitor, such as 1–10 µF, may also be added when the supply is noisy or located farther away.

Never leave an unused amplifier channel floating. Configure it as a voltage follower by connecting its output to its inverting input and tying the non-inverting input to ground or another valid reference voltage.

LM2904N Circuit Design and Calculations

The LM2904N can perform several analog signal-processing functions by using external resistors and capacitors. The following examples assume that the device operates within its input common-mode range, output-voltage limits, bandwidth, and load-current capability. A 0.1 µF ceramic capacitor should be connected close to its supply pins.

Non-Inverting Amplifier

A non-inverting amplifier applies the signal to the positive input. Resistor Rf connects the output to the inverting input, while Rg connects the inverting input to ground or a reference voltage. The voltage gain is:

For example, using R_f=40kΩ and Rg=10kΩ:

If the input is 0.2 V, the expected output is:

Vout=0.2V×5=1V

This circuit provides a positive output, meaning the output increases when the input increases.

Inverting Amplifier

An inverting amplifier applies the signal through R_into the inverting input. The non-inverting input is connected to ground in a dual-supply circuit or to a suitable reference voltage in a single-supply circuit. Its gain is:

With Rf=50kΩ and Rin=10kΩ:

An input of 0.2 V produces:

V_out=0.2V×(-5)=-1V

This example requires a negative supply because the output is below ground. In a single-supply design, the circuit must be biased around a reference voltage instead.

Voltage Follower

A voltage follower connects the output directly to the inverting input and applies the signal to the non-inverting input. Its voltage gain is one:

Av=1

Therefore, a 1 V input ideally produces a 1 V output. Although the circuit does not increase voltage, it provides high input impedance and low output impedance. This prevents a high-resistance sensor from being heavily loaded while allowing its signal to drive the next circuit stage.

The input and output must still remain within the LM2904N’s permitted voltage ranges. The follower can operate close to ground, but neither its input nor output can normally reach the positive supply rail.

Active Low-Pass Filter

A first-order active low-pass filter can be created using an input resistor-capacitor network followed by an LM2904N voltage follower. It passes low-frequency signals while reducing high-frequency noise. Its cutoff frequency is:

Using R=15.9kΩ and C=0.1μF:

Signals below approximately 100 Hz pass with little attenuation. The output falls by 3 dB at the cutoff frequency and then decreases at about 20 dB per decade above it. When powered from a single supply, an AC signal may need to be biased around a suitable reference voltage.

Sensor Signal-Conditioning Circuit

Suppose a sensor produces 0.1–0.8 V, but a 3.3 V ADC needs a larger signal. The LM2904N can operate from 5 V and amplify the sensor output using a non-inverting circuit. Choose Rf=30kΩ and Rg=10kΩ:

The resulting output range is:

V_(out(min))=0.1V×4=0.4V

V_(out(max))=0.8V×4=3.2V

This range remains below the 3.3 V ADC limit and sufficiently below the LM2904N’s 5 V positive supply. Resistor tolerances, offset voltage, sensor faults, and ADC input protection must also be considered before using the circuit in a final design.

Application Curve of LM2904N

The curve shows how the LM2904N output voltage changes as the input voltage increases in a non-inverting amplifier. In the rising linear region, the circuit follows the gain equation Av=1+R2/R1. With the resistor values shown, the gain is 101, so a small increase in input voltage produces an output approximately 101 times larger without reversing its polarity.

The output stops increasing when it approaches the LM2904N’s maximum output-swing limit. Although the circuit uses a +5 V supply, the output cannot normally reach +5 V because the LM2904N is not rail-to-rail. The flat part of the curve represents positive-output saturation, where further increases in input voltage no longer produce proportional output changes.

LM2904N Advantages and Limitations

LM2904N Advantages

The LM2904N consumes relatively little supply current and can operate from a single power source, reducing power-system complexity. Its input common-mode range includes ground, making it useful for processing low-level signals in single-supply circuits. It contains two independent op-amps, allowing two signal-processing functions to be implemented with one IC. The LM2904N is also inexpensive and widely available in a PDIP-8 through-hole package, which is convenient for breadboards, prototypes, repairs, and educational projects.

LM2904N Limitations

The LM2904N has modest bandwidth and slew rate, so it is unsuitable for high-frequency signals or outputs that must change rapidly. Its output current is limited and should not directly drive motors, relays, speakers, or other heavy loads. The input-offset voltage can also create noticeable errors when amplifying very small signals or using high closed-loop gain. In addition, the device is not rail-to-rail. Its input and output can operate near the negative rail, but they cannot normally reach the positive supply voltage. A modern rail-to-rail or precision op-amp may be more suitable for low-voltage, high-speed, or high-accuracy designs.

LM2904N Comparisons and Replacement Options

LM2904N vs LM358N

The LM2904N is generally better for industrial equipment because it supports temperatures from −40°C to +85°C. The LM358N is intended for the narrower commercial range of 0°C to +70°C, although it supports a higher maximum supply voltage. Both have similar offset accuracy and bandwidth, and both are available in PDIP-8 packages. Choose the LM2904N for environments with wider temperature changes and the LM358N for ordinary indoor circuits where its wider supply range is useful.

LM2904N vs LM2904B

The LM2904B provides the best overall electrical performance. It has a wider supply range, lower offset voltage, higher bandwidth, faster slew rate, improved output capability, internal EMI filtering, and a −40°C to +125°C temperature range. It is usually the better option for new designs requiring improved accuracy, faster signals, or high-temperature operation.

However, the TI LM2904B is available only in surface-mount packages. It cannot directly replace a PDIP-8 LM2904N on a through-hole board without an adapter or PCB modification. The LM2904N remains the better choice for breadboards, repairs, socketed ICs, and existing through-hole designs.

The LM2904B is the best overall performer for new surface-mount designs. The LM2904N is the best through-hole option for industrial environments, while the LM358N is suitable for low-cost commercial circuits operating within a controlled temperature range. Always verify the complete manufacturer part number because specifications and package options can vary.

Common LM2904N Circuit Problems

• Output stuck near a supply rail: This may occur when the input voltage difference is too large, feedback is disconnected, the required output exceeds its permitted range, or the device is operating open-loop. Check the input voltages, feedback network, and supply connections.

• Signal clipping: The LM2904N is not rail-to-rail. Clipping occurs when the circuit attempts to produce an output too close to the positive supply or beyond the available negative swing. Reduce the gain, input amplitude, or signal bias level.

• Oscillation or excessive noise: Long wiring, poor grounding, capacitive loads, inadequate PCB layout, or missing supply decoupling can make the output unstable. Shorten signal paths and separate sensitive inputs from noisy traces.

• Incorrect gain: Incorrect resistor values, resistor tolerances, wiring errors, source resistance, and output saturation can cause the measured gain to differ from the calculated value. Verify every feedback-network connection and resistor value.

• Output unable to drive the load: The LM2904N has limited output-current capability. A low-resistance or highly capacitive load can reduce the output swing, distort the signal, or cause instability. Use a buffer or driver stage for demanding loads.

• Input outside the common-mode range: The inputs can operate near ground but cannot normally reach the positive supply. Exceeding this range may produce an incorrect or unpredictable output.

• Missing bypass capacitor: Without local supply decoupling, power-line noise and sudden current changes may affect operation. Place a 0.1 µF ceramic capacitor close to pins 8 and 4.

• Floating unused amplifier: An unconnected channel can amplify electrical noise and cause unnecessary current changes. Configure it as a voltage follower and connect its non-inverting input to a valid reference voltage.

• IC installed in the wrong orientation: Reversing the PDIP-8 package connects the supply voltage to incorrect pins and may damage the device. Locate the notch or pin-one marker before installation.

Mechanical Dimension