Please consider this schematic of an electret microphone capsule:
While considering something similar for signal conditioning of a 20 mVpp, 3k impedance output from a piezo bender, I found that discrete JFETs seem to have declined in availability and popularity, and increased in price.
This leads to a suspicion that some other discrete component that works with low signal voltage, high impedance, bipolar signals has replaced the JFET for such purposes, where anything more than a single discrete component would be overkill. A MOSFET would require biasing beyond Vgs(th), that is my fall-back.
- What is the modern discrete alternative to the cheap-and-ubiquitous JFET?
I have considered the ALD110900 N-channel zero threshold MOSFET, but it is not cheap, not ubiquitous, and doesn't support gate voltage dropping below source.
The attraction of the JFET in applications such as the electret mic above is that it is a 3-terminal device, no regulated Vcc is required, zero-bias input signal is fed in, input modulates resistance regulates current, and it all works!
With integrated circuits like op-amps, that simplicity of implementation is lost.
Please note that the context below is not the question, it only indicates where the thought process started.
A high impedance (3 kOhm) bare piezoelectric sensor (piezo bender) is located in an underwater, hermetically sealed hydrophone at the remote end of a 5 to 15 to meter long single-core shielded cable. The 20mV to 70 mV peak-to-peak signal is getting swamped along the line due to the location being extremely EMI-rich (industrial environment). At the operator end is a hand-held, battery operated readout device that is attached to the sensor cable when required. The signal needs reinforcement in some way, for getting it back to the hand-held unit.
Some minimal circuitry can be added at the sensor end if needed, and the handheld end circuitry can be modified as required. Modification of enclosures or cabling is not an option. Volumes are 10-20 units, so a 1ku order on parts is not an option.
- No option for adding power supply wire to sensor enclosure.
- Limited power: 4xAA battery operated readout device.
- Sensor enclosure too small for AA cell or much circuitry.
- Sensor enclosure cannot be retrieved more than once a year or so for battery replacement etc.
We have switched from JFETs to depletion mode NFETs such as a BSS169.