Matching JFETs for piezo buffer preamp

[NateS]

As for the input caps - Zach's notes say if the piezo is isolated then they can be omitted. They're not necessary to work - though they're probably handy for HPF more than anything - but without a model of my actual pickup, I'm not sure what to put there. (I want this for a phantom powered endpin preamp to a a JJB-330 piezo. 15mm piezos, but no clue as to their capacitance). Hard to get more isolated that the underside of an acoustic guitar top though.

[MarkusBass]

That's true but you are talking about the real circuit and here we are talking about the simulation. Without proper model of the piezo transducer it will not work correctly.

Mark

[salvarsan]

AVR-based transistor testers are available on eBay and Amazon starting at $15-$20.
Based on the transistortester on github, they have varying degrees of decoration -- case and cables cost more, of course.
Original work is documented in a wiki on Mikrocontroller.net.

They infer the device pinout for you whether diode, BJT, FET, MOSFET, SCR, or triac.

Some are more informative than others but they all seem to minimally show pinout, symbol, characteristic current and voltage, or gate capacitance and Vth for MOSFETs.

They look like:



Having just sorted a slew of 2N5459 N-JFETs, I'm pretty satisfied with my $17 one.
They are tested with 470k gate resistor, 680 ohm drain resistor, grounded source.

Note, if you've sorted JFETs by Vp, pinchoff voltage, they are sorted by Idss because of precision limits.
On mine, Idss is reported at 1 decimal point while the Vp gets 2.
That's what you can reasonably expect from a middling voltage reference, 1% resistors, and 10-bit ADCs.

[NateS]

One last thing about this circuit - I'm getting a bit more gain than I think I want. (+24db). The 200mv input will exceed +4dbu (it won't clip until the input hits about 800mV but it likely will overdrive the mixer inputs.)

I can trade some of that away with local feedback (R9/R12) but that seems as if it'd seriously degrade CMMR - something this circuit absolutely requires (it has no blocking of the phantom power - relying on CMMR exclusively to avoid its influence). Changing feedback via R4/R11 changes the gate bias and lowers input resistance. Of course I could just pad it down, but what's the best way to increase feedback without the above mention ill effects? Would it be good practice to bypass J3 drain to ground or will that not add anything to CMMR?

[NateS]

Well, that was counterintuitive... bypassing the current source makes PSRR worse. Increasing the individual source local feedback resistors makes PSRR better. Actually, as is it's incredibly tolerant of power supply noise (and I presume CMMR). Bringing the R9/R12 feedback resistors up to 1.2k drops the gain down to about 12db.

[diydidi]

You Can match them by tying gate to drain and connect this through a 10k resistor to 12v and then connect source to 0V. Measure for Vgs,