John Larkin recently pointed out the NE3508 and NE3509. They are really nice, steep Vgs versus Id and so on. Even better than the BF862 except for one major obstacle: At >>50c they are way out of league for many designs. High drool factor, followed by a depressed look at the BOM budget.
Question: Are there similar hotrod JFETs that are in the 10-20c range?
The BF862 is a rusty old tractor compared to the NEC hj parts. The hjfets have roughly 10 times the transconductance and 10 times less capacitance than the jfet, which is a 100:1 gain-bandwidth advantage. The Rds-on ratio is, like, 20:1.
Also, in my experience, the hj fets are much more repeatable than jfets. The Idss of the 862 is spec'd at 10-25 mA, which is actually good for a jfet. Spec-sheet ranges of Idss are 10:1 for some jfets!
In my business, fast time-domain stuff, we don't use jfets. Their poorly specified dc characteristics, and their low gain, make them pretty much useless, especially since I can buy 1.8 GHz opamps and 10 GHz mmics nowadays.
John
I was thinking more about oscillators that must work with really low supply voltage, a few hundred millivolts. The high gain of the NEC parts is really beneficial there while the speed would have to be muffled big time so they remain stable. Pricing is a serious problem though.
Jfets are good for HF front-ends and for oscillators, ald for high-impedance instrumentation. The hj fets are noisy at low frequencies, especially gate current noise. I don't know if they have the "dispersion" problem that mesfets typically have, namely changes in Gm at sub-MHz frequencies, or if they have the trapping-state problems that mesfets have.
I'm trying to do a "pin driver" that will swing from 0.25 to 5 volts p-p, with +-5 volt programmable offset, and produce an arguably square wave from dc to 1 GHz. This isn't a very cost-sensitive application.
John
In sci.electronics.components John Larkin wrote: ...
However the Gate leckage current isn't specified, with makes it hard to see if the part is usefull for high impedance work, like charge sensitive amplifiers.
Oh, the gates leak microamps. But multi-GHz signals don't exist at high impedances, so it's OK.
Properly tuned, uncooled, the 3509 has a noise figure of 0.4 dB at 2 GHz, about 29 Kelvins. Try that with a jfet!
John
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