BF245B / BF245C JFET - Suitable for audio?

GregH2 · 21st February 2011, 05:22 AM

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, 06:38 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, 06:57 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, 07:11 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, 07:33 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, 08:00 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,

21st February 2011, 05:22 AM	#1
GregH2 diyAudio Member Join Date: Oct 2009	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.

21st February 2011, 06:38 AM	#2
Mooly diyAudio Member Join Date: Sep 2007	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. __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*

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21st February 2011, 06:57 AM	#3
davidsrsb diyAudio Member Join Date: Dec 2005 Location: Kuala Lumpur	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.

21st February 2011, 07:11 AM	#4
Elvee diyAudio Member Join Date: Sep 2006	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.

21st February 2011, 07:33 AM	#5
mjf diyAudio Member Join Date: Jan 2009 Location: upper austria/near linz	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

21st February 2011, 08:00 AM	#6
GregH2 diyAudio Member Join Date: Oct 2009	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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