Jfet matching

[lowell]

Ok thanks for the in depth post Sir Geo.

I'll do what you suggest and report the results.

So... is it the ESR of an electrolytic that mitigates it's ability to snub uber high frequencies? Always been curious when I've seen circuits that have an average filter cap with smallish tantalum or ceramics bypassing them. They must be faster...or more efficient, less resistance in the higher frequencie bands. ?

[R.G.]

It's not so much the ESR as the ESL - equivalent series inductance.

If you look at electro cap datasheets, they sometimes have plots of impedance versus frequency, which starts at high impedance at DC and slopes down linearly with frequency, the classical Xc = 1/(2*pi*F*C). This bottoms out at some frequency, and that minimum is the ESR, the capacitive reactance and the inductive reactance actually cancelling at that frequency - which is the self resonance of the cap plus any lead lengths. From there, as frequency rises, the ESL causes the cap's impedance to rise as though the "cap" was just the ESR plus the ESL. It's not a cap at high frequencies, it's an inductor. This is from just wire inductance, and from the coiling of the aluminum foil.

If you put a ceramic cap in parallel, the ceramic has less capacitance, but far, far less ESR and ESL. So its minimum point is at a way higher frequency than the electro cap it bypasses.

Smaller value ceramics again get lower ESR and ESL, so for high frequency radio work, you may see both, say, a 0.1uF ceramic bypassed by a 0.01uF, bypassed by a 0.001uF ceramic. Actually, by the time you get to those frequencies, you put the bypass cap as close to the active device as you can physically get it because the inductance of the wires/PCB traces get big.

That's one reason SMD is so good for logic and radio: the ESL of an SMD device is vanishing small, as it has no leads at all.

[lowell]

RG. Thanks so much for that post.

What current jfet are you using in place of the 2n5485? They're becoming more and more difficult to find...and more $$. How do you typically go about finding a new make of a part...jfet...when one is phased out and becomes obsolete? I see them for about .49c online at Tayda Electronics. Is that a good price or is there a new make that works well at a better price point?