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The LF351 Op-Amp IC is a single JFET-input operational amplifier designed for analog circuits that need high input impedance, low input current, and good signal speed. Unlike basic bipolar op-amps such as the LM741, the LF351 uses a JFET input stage, so it can handle weak or high-resistance signals without loading the source too much. In this article, you will learn what the LF351 is, how its pinout works, what its datasheet values mean, how it performs in real applications, and how it compares with alternatives like the LM741 and TL071.
| Pin Number | Pin Name | Function |
| 1 | Offset Null | Used to adjust or reduce the input offset voltage. Usually connected with Pin 5 and a potentiometer when offset trimming is needed. |
| 2 | Inverting Input | The negative input terminal of the op-amp. The output signal is inverted when the input signal is applied here. |
| 3 | Non-Inverting Input | The positive input terminal of the op-amp. The output signal keeps the same phase when the input signal is applied here. |
| 4 | VCC− / V− | Negative power supply pin. In dual-supply circuits, this is usually connected to the negative voltage rail. |
| 5 | Offset Null | Used together with Pin 1 for offset voltage adjustment. It can be left unused if offset trimming is not required. |
| 6 | Output | Output pin of the op-amp. The amplified signal comes out from this pin. |
| 7 | VCC+ / V+ | Positive power supply pin. This provides the positive operating voltage for the IC. |
| 8 | NC | No internal connection. This pin is normally left unconnected. |
| Category | Specification | Value | Unit |
| Power Supply | Maximum supply voltage | ±18 | V |
| Input Rating | Maximum input voltage | ±15 | V |
| Differential input voltage | ±30 | V | |
| Input Accuracy | Input offset voltage | 3 typical, 10 max | mV |
| Input offset voltage drift | 10 | µV/°C | |
| Input Current | Input offset current | 5 typical, 100 max | pA |
| Input bias current | 20 typical, 200 max | pA | |
| Voltage Gain | Large signal voltage gain | 50 min, 200 typical | V/mV |
| Power Rejection | Supply voltage rejection ratio | 80 min, 86 typical | dB |
| Supply Current | No-load supply current | 1.4 typical, 3.4 max | mA |
| Input Range | Input common-mode voltage range | ±11 min, +15/-12 typical | V |
| Noise Rejection | Common-mode rejection ratio | 70 min, 86 typical | dB |
| Output Rating | Output short-circuit current | 10 min, 40 typical, 60 max | mA |
| Output Swing | Output voltage swing, RL = 2kΩ | ±10 min, ±12 typical | V |
| Output voltage swing, RL = 10kΩ | ±12 min, ±13.5 typical | V | |
| Speed | Slew rate | 12 min, 16 typical | V/µs |
| Rise time | 0.1 | µs | |
| Stability | Overshoot factor | 10 | % |
| Bandwidth | Gain bandwidth product | 2.5 min, 4 typical | MHz |
| Input Impedance | Input resistance | 10¹² | Ω |
| Audio Performance | Total harmonic distortion | 0.01 | % |
| Noise | Equivalent input noise voltage | 15 | nV/√Hz |
| Stability | Phase margin | 45 | Degrees |
| Thermal Rating | Junction-to-ambient thermal resistance, SO-8 | 125 | °C/W |
| Junction-to-ambient thermal resistance, DIP8 | 85 | °C/W | |
| Junction-to-case thermal resistance, SO-8 | 40 | °C/W | |
| Junction-to-case thermal resistance, DIP8 | 41 | °C/W | |
| Storage | Storage temperature range | -65 to +150 | °C |
| ESD Protection | Human body model | 500 | V |
| Machine model | 200 | V | |
| Charged device model | 1.5 | kV |
The LF351 works by comparing the voltage at its non-inverting input and inverting input. If the non-inverting input is higher, the output moves positive. If the inverting input is higher, the output moves negative. The small voltage difference between the two inputs is amplified into a larger output signal.
Its input stage uses JFET transistors, which give the LF351 very high input resistance and very low input bias current. This means it does not take much current from the signal source. Because of this, it is useful for high-impedance sensors, audio preamps, active filters, and signal-conditioning circuits.
Inside the IC, the signal passes through gain stages that increase the voltage level. The LF351 is internally compensated, so it can work in common feedback circuits such as voltage followers, inverting amplifiers, and non-inverting amplifiers without extra compensation parts.
The offset null pins allow small input offset errors to be adjusted. This helps reduce unwanted output error when both inputs should be equal.
The output stage delivers the amplified signal to the output pin. However, the LF351 is not rail-to-rail, so the output cannot swing fully to the positive or negative supply voltage. For best performance, it should be used with a suitable supply voltage, often in dual-supply analog circuits.
• JFET input stage - Provides very high input resistance and very low input bias current, making it suitable for weak or high-impedance signals.
• Wide bandwidth - The LF351 has a typical gain bandwidth product of 4 MHz, which helps it handle faster analog signals than older general-purpose op-amps.
• High slew rate - With a typical slew rate of 16 V/µs, it can respond quickly to changing input signals.
• Low input bias current - Its typical input bias current is very small, making it useful for sensor circuits and precision signal inputs.
• Internal frequency compensation - The IC is internally compensated, so it is easier to use in common amplifier circuits without extra compensation parts.
• Offset null adjustment - Pins 1 and 5 allow offset voltage trimming when better DC accuracy is needed.
• Good input impedance - The high input resistance helps prevent signal loss from high-resistance sources.
• Short-circuit protection - The output has protection against short-circuit conditions, improving device safety during fault situations.
• Suitable for analog signal circuits - It is commonly used in active filters, audio preamplifiers, oscillators, buffers, and signal-conditioning circuits.
The diagram shows two common LF351 application circuits: a square wave oscillator and a high-Q notch filter. These circuits use the LF351 as an active analog device, not just as a simple voltage amplifier. The LF351 works well here because it has a JFET input stage, high input impedance, wide bandwidth, and good speed.
In the square wave oscillator, the LF351 is powered by a dual supply of +15V and -15V. The capacitor CFcharges and discharges through the feedback resistor RF. As the capacitor voltage rises and falls, the LF351 switches its output between high and low levels, creating a square wave. The oscillation frequency is mainly set by RF and CF, using the formula:
In this example, the circuit produces a very low frequency of about 0.5 Hz.
In the high-Q notch filter, the LF351 is used to remove or reduce one specific unwanted frequency while allowing other frequencies to pass. The resistor and capacitor network connected to the input sets the notch frequency. The formula shown is fo=1/(2πR1 C1 ), and the example values produce a notch frequency of about 1 kHz. This type of circuit is useful for removing narrow-band noise, tone interference, or unwanted signal components.
These examples show that the LF351 is suitable for timing, filtering, and signal-conditioning circuits. To use it correctly, the power supply must match the circuit requirement, the feedback components must be chosen based on the target frequency, and the output load should not be too heavy. Since the LF351 is not rail-to-rail, its output will not reach the full supply voltage, so enough supply headroom is needed for proper operation.
The LF351 can be used in audio preamplifiers, tone control units, mixers, and basic equalizer stages. Its high input impedance helps accept weak audio signals without loading the source. It is suitable for general audio use, but newer low-noise op-amps may be better for high-end audio systems.
The LF351 is useful in systems that read signals from high-impedance sensors. It can help condition signals from light sensors, piezoelectric sensors, chemical probes, and other transducers before the signal is processed by another device.
The LF351 can be found in analog test equipment such as signal generators, waveform generators, frequency filters, and measuring instruments. Its wide bandwidth and good slew rate help it handle changing analog signals more effectively than older general-purpose op-amps.
The LF351 can be used in industrial control systems that need analog signal amplification or conditioning. It can help process control signals from sensors, feedback networks, and monitoring circuits before they are sent to controllers or display units.
The LF351 is suitable for equipment that needs frequency selection or noise reduction. It can be used in audio filters, notch filters, and analog signal paths where unwanted frequencies must be reduced while keeping the useful signal.
The LF351 is also common in electronics learning, laboratory experiments, and repair of older analog equipment. Because it is a single JFET-input op-amp in a standard 8-pin package, it is easy to understand, test, and replace in many legacy circuits.
| Category | LF351 | LM741 | TL071 |
| Op-amp type | Single JFET-input op-amp | Single bipolar-input general-purpose op-amp | Single JFET/FET-input low-noise op-amp |
| Input stage | JFET input | Bipolar transistor input | JFET/FET input |
| Typical use | Filters, sensor interfaces, audio preamps, signal conditioning | Basic amplifier circuits, learning circuits, older analog designs | Audio circuits, filters, preamps, signal conditioning |
| Input bias current | Very low, about 20 pA typical | Much higher, about 80 nA typical | Low, about 65 pA typical for standard TL07x versions |
| Slew rate | 16 V/µs typical | 0.5 V/µs typical | Up to 20 V/µs typical, depending on version |
| Gain bandwidth product | 4 MHz typical | Around 1 MHz class | 3 MHz to 5.25 MHz, depending on version and package |
| Input resistance | Very high, about 10¹² Ω | Lower, about 2 MΩ typical | Very high, about 1 TΩ |
| Noise performance | Good for general analog use | Not ideal for low-noise signals | Better choice for low-noise audio and signal circuits |
| Output swing | Not rail-to-rail | Not rail-to-rail | Not fully rail-to-rail, but modern versions have improved range |
| Supply voltage | 6 V to 32 V total operating range | Commonly used with ±15 V; can use single or dual supply | Modern TL07xH supports ±2.25 V to ±20 V or 4.5 V to 40 V |
| Best advantage | High input impedance with good speed | Simple, common, and useful for basic circuits | Low noise, fast response, and good audio performance |
| Main limitation | Not ideal for low-voltage rail-to-rail circuits | Slow, higher input current, older performance | Version differences must be checked before replacement |
| Best choice for modern use | Good for high-impedance analog circuits | Mostly for learning or legacy repair | Usually the better modern choice than LM741 |
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