The **LS840-2** (specifically the LS840 series manufactured by **Linear Systems**) is a high-performance **Monolithic Dual N-Channel JFET** (Junction Field-Effect Transistor). It is designed primarily for high-gain, low-noise differential amplification. --- ### 1. Key Technical Specifications The "2" in the part number usually refers to a specific grade of electrical characteristics (often related to Gate Threshold Voltage or Gate Leakage). | Parameter | Value / Range | | :--- | :--- | | **Type** | N-Channel Monolithic Dual JFET | | **Breakdown Voltage ($V_{GSS}$)** | -40V | | **Gate Leakage Current ($I_{G}$)** | < 10pA (Ultra-low) | | **Transconductance ($g_{fs}$)** | 1000 µS to 3000 µS | | **Input Capacitance ($C_{iss}$)** | ~5 pF | | **Package Options** | TO-71, SOIC-8, SOT-23 | --- ### 2. Core Electronic Features #### A. Monolithic Construction Unlike discrete JFETs paired together, the LS840-2 is built on a single silicon chip. This ensures: * **Thermal Tracking:** Both transistors stay at the same temperature, preventing offset drift. * **Tight Matching:** The characteristics ($V_{GS}$ and $I_{DSS}$) of the two transistors are nearly identical. #### B. Low Noise Performance The LS840-2 is optimized for low-frequency noise reduction. It is often used in sensitive sensor interfaces where "flicker noise" ($1/f$ noise) must be minimized. #### C. High Input Impedance With a gate leakage current in the picoampere range, it draws almost no current from the signal source, making it ideal for high-impedance transducers like pH probes or piezo sensors. --- ### 3. Common Pin Configuration (SOIC-8 / TO-71) In a standard dual package, the pins are typically assigned as follows: | Pin Number | Function | Description | | :--- | :--- | :--- | | 1 | **S1** | Source of JFET 1 | | 2 | **D1** | Drain of JFET 1 | | 3 | **G1** | Gate of JFET 1 | | 4 | **Substrate** | Usually connected to the most negative rail | | 5 | **G2** | Gate of JFET 2 | | 6 | **D2** | Drain of JFET 2 | | 7 | **S2** | Source of JFET 2 | --- ### 4. Applications The LS840-2 is frequently found in high-end precision instrumentation: 1. **Differential Input Stages:** Used as the first stage in Operational Amplifiers (Op-Amps) to provide high CMRR (Common Mode Rejection Ratio). 2. **Electrometer Amplifiers:** Measuring extremely small currents. 3. **Audio Pre-amplifiers:** High-fidelity audio equipment requiring low noise and high input impedance. 4. **Medical Instrumentation:** Buffering signals from biological sensors.