JFETs for 5ma CCS?

PhopsonNY · 3rd April 2003, 12:30 PM

Hi there,

Does anyone know what combination of Cascoded JFETS and resistor are needed to obtain a 5mA opamp Biasing into Class A.

I cannot seem to find out the exact JFET # to acheive this.

Thanks for all the help.

Cheers

SY · 3rd April 2003, 01:13 PM

Any JFETs with an idss of more than 5 ma will work. The exact resistor values will have to be determined experimentally for each FET pair, since idss can vary 5 to 1 for a given part number.

L Daniel Rosa · 4th April 2003, 03:47 PM

A quick look through my 1968 data book (I got it for 50 cents!) gives a few possibilities for JFETs. Some of the items for 5-10 mA Idss are 2n3822, 2n4221, mfe2093-mfe2095, mfe4002, mfe4005, mpf104, mpf152, mpf155, mfe2000. For 10-20 mA Idss there are 2n4222, mfe4003, mfe4006, mpf105, mpf 153, mpf156, 2n3823, 2n4223, 2n4224, 2n4416, mfe2001, mfe2002, mpf102, mpf107. A lot of these may be hard to locate and many of them are in cans, but they did exist at one time.

With a 5mA target current, have you consedered using a CCS made of BJTs?

Audiofanatic · 4th April 2003, 07:33 PM

Quote:

With a 5mA target current, have you consedered using a CCS made of BJTs?

Hi Daniel Rosa,

Do you prefer a BJT to a Fet? And if yes, why?

Audiofanatic

L Daniel Rosa · 4th April 2003, 07:56 PM

Audiofanatic,
I use BJTs because that's what I have. If I were to try to keep a reasonable assortment of FETs I'd have to buy a second garage. As for the strengths and weaknesses, there's a a wide overlap where it may be difficult to choose one over the other. What BJTs offer are: easy availability in higher voltages; higher current ratings; for a given device a more independant dropout voltage with respect to current.

Fred Dieckmann · 4th April 2003, 09:48 PM

http://www.vishay.com/docs/70596/70596.pdf

When cascoding I use resistor votage divider on gate of upper jfet to get 5 to 7 volts on bottom JFET for best Goss.

lineup · 20th December 2008, 05:58 AM

Quote:

Originally posted by Fred Dieckmann
http://www.vishay.com/docs/70596/70596.pdf
When cascoding I use resistor votage divider on gate of upper jfet
to get 5 to 7 volts on bottom JFET for best Goss.

Good PDF with JFET current sources. Thanks

I see 2N4416 in list above.
Is it any good for preamplifiers?
Which one is better: 2N4416 or Fairchild PN4416 ... ???

bear · 24th December 2008, 03:55 AM

They used to make purpose made "diodes" specd that way... Siliconix used to make 'em... maybe Vishay has the line now...

3rd April 2003, 12:30 PM	#1
PhopsonNY diyAudio Member Join Date: Feb 2002 Location: New York	JFETs for 5ma CCS? Hi there, Does anyone know what combination of Cascoded JFETS and resistor are needed to obtain a 5mA opamp Biasing into Class A. I cannot seem to find out the exact JFET # to acheive this. Thanks for all the help. Cheers

3rd April 2003, 01:13 PM	#2
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	Any JFETs with an idss of more than 5 ma will work. The exact resistor values will have to be determined experimentally for each FET pair, since idss can vary 5 to 1 for a given part number. __________________ If there's a sucker born every minute, where do the rest of them come from?

4th April 2003, 09:48 PM	#6
Fred Dieckmann diyAudio Retiree Join Date: Oct 2002 Location: Spain or the pueblo of Los Angeles	Jfet current sources http://www.vishay.com/docs/70596/70596.pdf When cascoding I use resistor votage divider on gate of upper jfet to get 5 to 7 volts on bottom JFET for best Goss.

24th December 2008, 03:55 AM	#8
bear expert in tautology diyAudio Member Join Date: Apr 2002 Location: New York State USA	They used to make purpose made "diodes" specd that way... Siliconix used to make 'em... maybe Vishay has the line now... __________________ _-_-bear http://www.bearlabs.com [...2SJ74 Toshiba bogus asian parts - beware! ] -- Btw, I don't actually know anything, FYI --

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4th April 2003, 03:47 PM	#3
L Daniel Rosa diyAudio Member Join Date: Jun 2002 Location: Bellingham WA	A quick look through my 1968 data book (I got it for 50 cents!) gives a few possibilities for JFETs. Some of the items for 5-10 mA Idss are 2n3822, 2n4221, mfe2093-mfe2095, mfe4002, mfe4005, mpf104, mpf152, mpf155, mfe2000. For 10-20 mA Idss there are 2n4222, mfe4003, mfe4006, mpf105, mpf 153, mpf156, 2n3823, 2n4223, 2n4224, 2n4416, mfe2001, mfe2002, mpf102, mpf107. A lot of these may be hard to locate and many of them are in cans, but they did exist at one time. With a 5mA target current, have you consedered using a CCS made of BJTs?

4th April 2003, 07:56 PM	#5
L Daniel Rosa diyAudio Member Join Date: Jun 2002 Location: Bellingham WA	Audiofanatic, I use BJTs because that's what I have. If I were to try to keep a reasonable assortment of FETs I'd have to buy a second garage. As for the strengths and weaknesses, there's a a wide overlap where it may be difficult to choose one over the other. What BJTs offer are: easy availability in higher voltages; higher current ratings; for a given device a more independant dropout voltage with respect to current.

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