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Old 3rd April 2003, 12:30 PM   #1
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Default 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
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Old 3rd April 2003, 01:13 PM   #2
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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.
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Old 4th April 2003, 03:47 PM   #3
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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?
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Old 4th April 2003, 07:33 PM   #4
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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?

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Old 4th April 2003, 07:56 PM   #5
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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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Old 4th April 2003, 09:48 PM   #6
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Default 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.
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Old 20th December 2008, 05:58 AM   #7
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Default Re: Jfet current sources

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 ... ???
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Old 24th December 2008, 03:55 AM   #8
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They used to make purpose made "diodes" specd that way... Siliconix used to make 'em... maybe Vishay has the line now...
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