JFETs which are intended for use with electret microphone capsules integrate an additional diode between the gate and source terminals as shown in Figure 7-1, labeled D_G. The primary purpose of this diode is to help keep V_GS close to 0V when a source is AC-coupled directly to the gate. When a voltage source is applied between the drain and source of the JFET, as shown in Figure 7-1, a very small leakage current flows through the drain to gate junction of the JFET (I_L1). If this leakage is not compensated, the gate voltage increases until the JFET's gate-to-source junction is forward-biased enough to equalize the current. Forward-basing the gate-to-source junction has several negative consequences including reduced input resistance and increased input capacitance. The gate diode, D_G, offers an additional leakage pathway (I_L2) to compensate for the drain-to-gate leakage (I_L1), holding the V_GS voltage closer to 0V, and maintaining high input impedance.

Figure 7-1 JFE2325 Input Leakage Pathways

Figure 7-2 shows the JFE2325 gate current as a function of V_GS. Very high input impedance is maintained for VGS values within ±100mV. In normal operation, with an AC-coupled input signal source, V_GS settles to approximately 50mV. Voltages beyond ±100mV cause the gate current to increase substantially.

Figure 7-2 JFE2325 Gate Current vs. Gate-to-Source Voltage