SLPU010 September   2022 JFE2140

 

  1.   Abstract
  2.   Trademarks
  3. 1Overview
    1. 1.1 JFE2140 Overview
    2. 1.2 JFE2140EVM Overview
      1. 1.2.1 Kit Contents
    3. 1.3 Related Documentation
    4. 1.4 Evaluation Module Limitations
    5. 1.5 Electrostatic Discharge Caution
  4. 2Getting Started
    1. 2.1 Power Supplies
    2. 2.2 Input
    3. 2.3 Output
    4. 2.4 Capacitors
  5. 3Application Circuit
    1. 3.1 Ultra-Low-Noise Preamplifier
  6. 4Schematic, PCB Layout, and Bill of Materials
    1. 4.1 Schematic
    2. 4.2 PCB Layout
    3. 4.3 Bill of Materials

3.1 Ultra-Low-Noise Preamplifier

Figure 3-1 shows a simplified schematic of the EVM for the purpose of this application circuit. Figure 4-1 shows the full default configuration of the JFE2140EVM. The EVM operates on a split supply of VDD = 5 V and VSS = –5 V. The output voltage, Vout, settles to approximately midsupply ±2 V.

Figure 3-1 Preamp With JFE2140 Front End in a Closed-Loop Circuit

Figure 3-1 shows that tail current ITail ≅ 2 mA. This figure also shows that resistors R37 and R15 form the beta feedback network that provides a closed-loop gain of 1001 V/V, or approximately 60 dB. Figure 3-2 shows the result. The high gain provided by the EVM is designed for small-signal amplification, but can also be used to measure the low VGS mismatch of the JFE2140. A VGS mismatch of 1 mV results in a dc output voltage of 1 V.

Figure 3-2 Gain vs Frequency

Figure 4-1 shows an optional integrator circuit using the OPA145 that helps reduce VGS mismatch. The integrator circuit monitors both gates of the JFE2140 and feeds back a current to one drain. The integrator circuit path uses components U3A, R39-R42, R44, R45, and C16–C18. Make sure that resistor R42 is large enough to maintain the dc bias voltage of the drain on JFET Q1B. The EVM default configuration uses SMA connector J8 for single-in and single-out signal amplification. XLR connectors Vdiff_In and Vdiff_Out are provided as an option for differential-signal amplification. When using the EVM for differential-signal amplification, remove composite amplifier U1A and resistors R37 and R15. Component footprints R18, R14, and C4 are provided for the second input. A path for earth ground is provided through resistor R43.