9 Application and Implementation

Note

Information in the following applications sections is not part of the TI component specification,

and TI does not warrant its accuracy or completeness. TI’s customers are responsible for

determining suitability of components for their purposes, as well as validating and testing their design

implementation to confirm system functionality.

9.1 Application Information

9.1.1 Input Protection Diodes

The JFE150 features diodes that are used to help clamp voltage surges that can occur on the input sensor

to the gate. The diodes are connected between the gate and two separate pins, VCL and VCH. The clamping

mechanism works by steering current from the gate into the VCL or VCH nodes when the voltage at the gate

is less than VCL or greater than VCH. Figure 9-1 shows an example of a microphone input circuit where a dc

blocking capacitor operates with a large dc voltage. When the microphone input is dropped or shorted, the dc

blocking capacitor discharges into the VCL or VCH nodes, thus helping eliminate large signal transient voltages

on the gate. There are also clamping diodes from the drain and source to VCL and VCH, respectively. The

clamping diodes can withstand high surge currents up to 200 mA for 50 ms; however, limit dc current to less than

20 mA.

JFE150

RG

10 …F

10 …F

48 V

RB

C

DC

iG

6.8 kŸ

R

L

VCH

D

S

VCL

G

Figure 9-1. JFE150 Clamping Diode Example

Figure 9-1 shows an example of configuring the diode clamp to protect the JFET against overvoltage in a

phantom-powered microphone circuit. Phantom power typically delivers 48 V through a 6.8-kΩ pullup resistor to

a microphone or dynamic load. If the microphone is disconnected, dc blocking capacitor CDC can be biased up to

48 V. If the input to the capacitor is then shorted to ground (shown by the switch in Figure 9-1), the gate voltage

can exceed the absolute maximum rating for VGS. In this case, the blocking diode is used, along with current

limiting resistors RG and RL, to clamp the gate voltage to a safe level. Be aware that the thermal noise of RG

couples directly into the gate input; therefore, make sure to minimize the resistance of RG.

The clamping diodes are not required for operation. The VGS voltage can withstand –40 V, so clamping is not

required if the VGS voltage is kept greater than this limit. If the diodes are not needed, leave the VCL and VCH

nodes floating.

Most previous-generation JFET devices featured only three pins (gate, source, and drain). For these devices,

the gate pin is in the same physical location as the VCL pin on the JFE150. To test the JFE150 in a three-pin

socket, short pin 2 of the JFE150 (VCL) to pin 3 (G). When the devices are connected with pin 2 shorted to pin 3,

the diode from VCL is shorted out and cannot provide any clamping protection. The input capacitance (CISS) also

increases by 1 pF; see Figure 6-12.

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JFE150

SLPS732B – JUNE 2021 – REVISED APRIL 2023

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