Junction field effect transistors (JFETs) are commonly used as an input stage in high-input-impedance, low-noise designs in audio, SONAR, vibration analysis, and other technologies. The JFE2140 is a new generation JFET device that offers very low noise performance at the lowest possible current consumption in high-input-impedance amplifier designs. The JFE2140 is manufactured on a high-performance analog process technology, giving tighter process parameter control than a standard JFET.

Designs that feature operational amplifiers (op amps) as the primary gain stage are common, but these designs are not able to achieve the lowest possible noise as a result of the inherent challenges and tradeoffs required from a full operational amplifier design. Noise in JFET designs can be evaluated in two separate regions: low-frequency flicker noise and wideband thermal noise. Flicker, or 1/f noise, is extremely important for systems that require signal gain at frequencies less that 100 Hz. The JFE2140 achieves extremely low 1/f noise in this range. Thermal noise is noise in the region greater than 1 kHz and depends on the gain, or gm, of the circuit. The gm is a function of the drain-to-source bias current; therefore, thermal noise is also a function of drain-to-source bias current. Figure 6-14 shows both 1/f and thermal noise with multiple bias conditions measured using the circuit shown in Figure 7-1.

Noise is typically modeled as a voltage source (voltage noise) and current source (current noise) on the input. The 1/f and thermal noise can be represented as voltage noise. Current noise is dominated by current flow into the gate, and is called shot noise. The JFE2140 features extremely low gate current, and therefore, extremely low current noise. Figure 6-15 shows how source impedance on the input is the dominant noise source. In nearly all cases, noise created as a result of current noise is negligible.