8 Detailed Description

8.1 Overview

The JFE150 is an ultra-low noise JFET designed to create low-noise gain stages for very high output impedance

sensors or microphones. Advanced processing technology gives the JFE150 extremely low-noise performance,

a high gm/CISS ratio, and ultra-low gate-current performance. Input protection diodes are integrated to clamp

high-voltage spurious input signals without the need for additional input diodes that can add leakage current

or distortion-creating non-linear capacitance. The JFE150 provides a next-generation device to implement low-

noise amplifiers for piezoelectric sensors, transducers, large-area condenser microphones, and hydrophones in

small-package options.

8.2 Functional Block Diagram

VCH

VCL

G

S

D

8.3 Feature Description

8.3.1 Ultra-Low Noise

Junction-gate 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 JFE150 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 JFE150 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 JFE150 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 6-9 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 JFE150 features extremely low gate current, and therefore, extremely low

current noise. Figure 6-10 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.

8.3.2 Low Gate Current

The JFE150 features a maximum gate current of 10 pA at room temperature, making the device an excellent

choice for maximizing the gain and dynamic range from extremely high impedance sensors. Additionally, any

noise contributions as a result of gate current are minimized because of the negligible shot noise at low current

levels. As with all JFET devices, when the drain-to-source voltage increases, the gate current also increases.

Keep the drain-to-source voltage to less than 5 V for the lowest gate input current operation.

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JFE150

SLPS732B – JUNE 2021 – REVISED APRIL 2023

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Product Folder Links: JFE150