NJM2068M

The NJM2068M is a high-performance dual operational amplifier from NJR (New Japan Radio, now Nisshinbo Micro Devices). Featuring JFET input stages, this dual op-amp offers very low input bias currents (in the picoampere range) and high input impedance, making it particularly suitable for high-impedance signal source applications such as photodiode amplifiers, piezoelectric sensors, and other circuits where input loading must be minimized. Compared to general-purpose dual op-amps like the 4558, the NJM2068 provides superior performance with a wider gain-bandwidth product (8 MHz vs 2-3 MHz), higher slew rate (5 V/µs vs 1-2 V/µs), and lower noise, while maintaining a compatible pinout for drop-in upgrades. The ‘M’ suffix indicates the DMP-8 (SOP-8) surface-mount package. The device operates from dual supplies of ±4V to ±18V or single supplies of 8V to 36V, providing flexibility for a wide range of applications. With low total harmonic distortion and excellent noise performance, the NJM2068M is particularly well-suited for audio applications including preamplifiers, headphone amplifiers, active equalizers, and tone control circuits. Its JFET inputs also make it ideal for high-impedance sensor interfaces and integrator circuits where low bias current is critical for accuracy.

The NJM2068M is a dual JFET-input operational amplifier that operates on classic op-amp principles with JFET differential input stages: 1) Input Stage: Each amplifier channel uses a pair of JFET (Junction Field-Effect Transistor) devices as the differential input pair. JFETs provide extremely high input impedance and very low input bias current (picoampere range), making these op-amps ideal for high-impedance signal sources. The differential pair converts the voltage difference between the two inputs into a current difference. 2) Gain Stage: The signal from the input stage is amplified by a high-gain intermediate stage, typically using a common-emitter BJT amplifier with an active load. This stage provides most of the op-amp’s open-loop gain. 3) Output Stage: A push-pull class AB output stage provides low output impedance and can drive moderate load impedances. The output stage is designed to provide wide output voltage swing, typically within a couple of volts of the supply rails. 4) Frequency Compensation: Internal frequency compensation ensures stability in closed-loop operation. The 8MHz gain-bandwidth product means that as closed-loop gain increases, bandwidth decreases proportionally (GBW = gain × bandwidth = constant). 5) Bias Circuitry: Internal bias circuits provide stable reference currents for all stages, ensuring consistent performance across supply voltage and temperature variations. 6) Operating Modes: As a voltage-feedback op-amp, the NJM2068 can be configured in various standard op-amp circuits: inverting amplifier, non-inverting amplifier, voltage follower, integrator, differentiator, active filter, summing amplifier, and comparator (though not optimized for that use). The JFET input means that input bias current is extremely low and is not significantly affected by common-mode voltage, which is beneficial for high-impedance circuits like integrators and charge amplifiers.

• Audio preamplifiers and phono stages (MM/MC cartridges)
• Active filters, equalizers, and tone control circuits
• High-impedance sensor signal conditioning (piezo, photodiode)
• Instrumentation amplifiers and precision analog circuits
• Servo error amplifiers and control loops
• Headphone amplifiers and line drivers
• Low-noise analog front-ends for measurement
• Professional audio mixing consoles and studio equipment