2N5457 and J201 replacements

[impycat1]

PF5102 is a direct replacement for the J201. Still available.

[WormBoy]

PF5102 is a direct replacement for the J201. Still available.

The Idss of that one is much, much higher ... even though Vgs is similar. I would not count on it as a direct replacement in general.

[BrianS]

/quote]
Depends on the circuit and how you define 'good'  . For a simple JFET buffer it might not matter much, but for a boost or overdrive ... An out of spec JFET will likely not give you the sound that the designer intended for the circuit, and it might not even bias properly with the resistors/trimmers that are in the design. There is a reason that a J201 will sound different in many circuits than a 5457, and that's because of specs  .

Makes perfect sense.  Thanks. 

[reddesert]

Please help me out with my understanding of in spec/out of spec.  I always thought that as long as a build didn't call for matched sets then as long as the transistor wasn't dead it was good to use.  Is this assumption on my part wrong? 

For a crude explanation, the V_gs is the voltage of gate below source at which the JFET begins to conduct, and this can be from about -0.5 to -several volts depending on type. And the Idss is the current through the drain and source resistors when the gate-source voltage is zero - this is typically from <1 to several milliamps depending on type. See the plot I posted on the previous page for some typical values.

Because the drain and source resistors are usually a few Kohms, the voltage drop across these resistors is Idss*R = a few volts.

Because we are working with low-voltage circuits with a supply that is often 9 V, there isn't much headroom - to bias a JFET amp stage into operating range, you want the drain voltage to be about 4-6V above ground, for example, and the drop across the drain resistor should be 9V minus that, so 3-5 Volts.   What that means is, as WormBoy said, if you use a JFET that has too large I_dss, you might not be able to bias it properly (unless you make the drain resistance very small, which reduces the gain of the stage). 

And if the JFET has a value of V_gs that is different from the design spec, it may not conduct when you want it to - this would mess up phaser stages, for example, and is why JFETs need to be matched for phasers. An amp stage doesn't need FET matching, but it still needs a gain  as a function of input voltage that is similar to what the designer intended.

AFAIK this need to operate at low voltage is why pedal designs tend to use J201 and 2N5457, because those types have some of the smallest Vp and Idss.