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GregH2 21st February 2011 06:22 AM

BF245B / BF245C JFET - Suitable for audio?
 
Hi All,

I'm just interested to know how well suited these JFETS are to audio use? I note they are sold as a "VHF amplifier" rather than a "switcher" or "chopper" so I imagine they are pretty good??

What is the effect of having interchangeable source and drain pins? The datasheet says the gate is static sensitive which is not the norm for JFETS as far as I know, so I guess that is one "side effect".

Also what static protection is needed? A 12v g-s zener? Or a g-d zener or both?

Anyway, would love your input and listening impressions!

Cheers,

Greg.

Mooly 21st February 2011 07:38 AM

The interchangeability of D and S is common to most JFET's (such as 2N3819) as they are totally symetrical. That always puzzled me... identifying them, so I asked Bob Pease of National Semiconductor who gave me the info.

I suspect the data sheet (was it Philips ?) is just being over cautious as JFET's are fairly immune to static in normal handling. I guess being an HF device means a narrow gate channel and walking over a nylon carpet and discharging a few 10's of Kv into the gate probably isn't a good idea but for normal handing... no problem.

No zeners are needed... the addition of any would ruin the characteristics anyway (junction capacitance). Just make sure the device is run within its ratings.

davidsrsb 21st February 2011 07:57 AM

In the jfet world "chopper" or "switcher" hints at a part with low Rds ON of 10R or less, fairly high Vgs off and capacitance and low gm.

Elvee 21st February 2011 08:11 AM

The BC264 is the corresponding LF Fet, but BF's also work well for audio:
BC264A datasheet pdf datenblatt - NXP Semiconductors - N-CHANNEL SILICON FIELD-EFFECT TRANSISTORS ::: ALLDATASHEET :::

These Fets are symetrical, but if you use them with the pins as they are labelled, you'll minimize the feedback capacitance.

mjf 21st February 2011 08:33 AM

hello.
the bf 244/245/256 jfet are not "real" audio transistors (like 2sk170 or so.....).
but in former times they were often used in the input stage of guitar amps and so on.their voltage gain is rather low,around 10x..............
greets

GregH2 21st February 2011 09:00 AM

Great, thank you everyone. Will try to track down some audio specific types. The BC264 looks good, but not overly common. I need something with at least a 30v rating and preferably something digikey stock. Any suggestions?

Greg,


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