jfet without biasing

noxworld · 1st July 2011, 02:47 PM

I have a strange question:
Somebody tell me that jfet can be used without any biasing in small signal amplifer,so very low noie can obtained.
is it true?
thanks!

AndrewT · 1st July 2011, 03:01 PM

The input can bias the jFET to zero Vgs. This requires that the jFET be run at exactly 100% of it's Idss.

noxworld · 1st July 2011, 03:40 PM

So what is the advantage and disadvantage of such method?

h_a · 1st July 2011, 03:50 PM

You mean no source resistor? Sure that's possible, though only for small signals (forward biasing).

Hannes

satoru · 2nd July 2011, 10:36 AM

The noise of JFET goes down proportional to the square root of the current. Higher the current, lower the noise. On the other hand, the leakage current goes up significantly higher when the JFET is biased positively. Practically saying, you better limit the positive bias to less than 0.1V to for obtaining negligible amount of leakage current for high gm JFETs. So, in other words, when you have less than 0.1 Vp-p input signal, a zero biased JFET can be used. I have posted a plot of leakage current of positively biased 2SK170 somewhere in the forum. What kind of amplifier are you designing?

Have fun!
satoru

1st July 2011, 02:47 PM	#1
noxworld diyAudio Member Join Date: Jun 2011	jfet without biasing I have a strange question: Somebody tell me that jfet can be used without any biasing in small signal amplifer,so very low noie can obtained. is it true? thanks!

1st July 2011, 03:01 PM	#2
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	The input can bias the jFET to zero Vgs. This requires that the jFET be run at exactly 100% of it's Idss. __________________ regards Andrew T.

1st July 2011, 03:50 PM	#4
h_a diyAudio Member Join Date: Sep 2006 Location: Graz, Austria	You mean no source resistor? Sure that's possible, though only for small signals (forward biasing). Hannes __________________ fresh matched IRFP240/IRFP9240 fets \|\| AlephJ/JX-kits F5 transistor kits \|\| Burning Amp BA-1/2/3 transistor kits

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
FS: PreAmp with Complimentary JFET Diff input & JFET ouput	USAcircuits	Vendor's Bazaar	12	21st June 2011 11:02 AM
Can I replace BF245C JFET with J201 JFET?	bigmike216	Parts	2	7th December 2005 06:50 AM
low-biasing or high biasing in Class-AB amps	Workhorse	Solid State	124	18th October 2005 04:45 AM
Biasing	Wyatt Turk	Tubes / Valves	10	1st October 2004 02:11 PM


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

1st July 2011, 03:40 PM	#3
noxworld diyAudio Member Join Date: Jun 2011	So what is the advantage and disadvantage of such method?

2nd July 2011, 10:36 AM	#5
satoru diyAudio Member Join Date: Jan 2007 Location: Berkeley, California	The noise of JFET goes down proportional to the square root of the current. Higher the current, lower the noise. On the other hand, the leakage current goes up significantly higher when the JFET is biased positively. Practically saying, you better limit the positive bias to less than 0.1V to for obtaining negligible amount of leakage current for high gm JFETs. So, in other words, when you have less than 0.1 Vp-p input signal, a zero biased JFET can be used. I have posted a plot of leakage current of positively biased 2SK170 somewhere in the forum. What kind of amplifier are you designing? Have fun! satoru

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read