Then what are the source and drain voltages?kingkorg said:
Yes, I stand by it. Most FET's need a source resistor, unless their gate is negatively biased.
Mic Biasing
- Thread starter fredwreck
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Help Support GroupDIY Audio Forum:
"Almost all JFETs are what's known as 'depletion mode' devices. This means that they conduct with no gate voltage (typically gate shorted to source or at the same potential). This is the maximum current region, and usually must be avoided for linear operation. The FET is biased off by applying a negative voltage to the gate with respect to the source, and they can be biased in exactly the same way as a valve. "
Source: https://sound-au.com/articles/fet-applications.htm , emphasis mine.
Absolute gate voltage (ie. with respect to ground) is NOT important or relevant. What IS, is the gate-to-source voltage, ie the RELATIVE voltage between them (which will change depending on the source resistor, drain resistor, transconductance and possibly / likely some other parameters).
Source: https://sound-au.com/articles/fet-applications.htm , emphasis mine.
Absolute gate voltage (ie. with respect to ground) is NOT important or relevant. What IS, is the gate-to-source voltage, ie the RELATIVE voltage between them (which will change depending on the source resistor, drain resistor, transconductance and possibly / likely some other parameters).
kingkorg said:
+1 to what Khron wrote.kingkorg said:
I would just add that you need to make sure the gate-source voltage needed for proper operation is not too high, just because the whole circuit operates on low voltage. Since source bias is used in most cases, increasing the source voltage also reduces the available voltage for the FET, which results in loss of headroom. FET's are available in several grades; you want the lowest Idss.
Matador
Well-known member
If it helps, my experience has been that when contemplating different JFET's, it's important to draw the intended load line on a typical VDS/IDS plot, then consider how that same line will look on the replacement JEFT. Often, tweaking of drain AND source resistances is in order.
To make things more difficult, JFET parameters across a handful of the SAME DEVICES can span a 300% range across common parameters like gate-source off voltage, IDSS. For example, for the J305 device, VGS(off) can span from -0.5V to -3V, and IDSS can span from 1mA to 8mA! For the 2N3819, VGSoff can be anywhere between -0.2V to -8V, and IDSS can be anywhere between 2mA and 20mA! This often means that you need to buy a handful and measure the one that will work best in your intended circuit.
To make things more difficult, JFET parameters across a handful of the SAME DEVICES can span a 300% range across common parameters like gate-source off voltage, IDSS. For example, for the J305 device, VGS(off) can span from -0.5V to -3V, and IDSS can span from 1mA to 8mA! For the 2N3819, VGSoff can be anywhere between -0.2V to -8V, and IDSS can be anywhere between 2mA and 20mA! This often means that you need to buy a handful and measure the one that will work best in your intended circuit.
Where would you inject the signal on a 2sk170? And where would I put the osciloscope probe to see the signal?It might make things easier if you can open up a DAW and use an oscilloscope plugin on a channel assigned to the input where the mic is connected. If you take this route, make sure to also keep an eye on the input level and reduce the preamp gain so you don't clip the input / ADC
The idea is to start with "a" signal amplitude, raise it until either the top or the bottom of the sine gets clipped, adjust the bias to "unclip" it, then rinse and repeat until both the top and bottom of the test-sine get clipped at the same time, roughly by the same amount.
The idea was to remove the need for a physical oscilloscope completely. Hence the "open up a DAW and use an oscilloscope plugin".
Signal goes into the JFET gate (where the mic capsule would be connected).
I know, but I want to test out my oscilloscope 
. I made a cable that goes from the headphone out, to an alligator clip, so ill clip that on the JFET gate. Where would I put my oscilloscope probe?
. I made a cable that goes from the headphone out, to an alligator clip, so ill clip that on the JFET gate. Where would I put my oscilloscope probe?You'll probably want at least two clips coming off that cable - one for the signal (going into the JFET gate, preferably through a DC-blocking capacitor), and one for the ground connection.
Scope probe ground clip to (circuit) ground, and you'll wanna look at the signal coming off the JFET drain pin.
Scope probe ground clip to (circuit) ground, and you'll wanna look at the signal coming off the JFET drain pin.
what size capacitor would you recommend my good friend ?
