Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Parts Where to get, and how to make the best bits. PCB's, caps, transformers, etc.

How high is the input impedance of 'a' jfet transistor?
How high is the input impedance of 'a' jfet transistor?
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
Reply
 
Thread Tools Search this Thread
Old 14th September 2006, 02:32 PM   #1
Hans L is offline Hans L  Netherlands
diyAudio Member
 
Hans L's Avatar
 
Join Date: Apr 2004
Location: Planet Earth
Default How high is the input impedance of 'a' jfet transistor?

What kind of value can I presume a jfet transistor has for its input impedance? Tens of MOhms? GOhms? I've searched datasheets, but apparently the input impedance (which naturally is presumed 'very high' for jfet) isn't stated. Can it be determined from other specs in the datasheet? For instance via the Zero-Gate Voltage Drain Current? I'm currently testing (read: learning) with the 2n5459 (http://www.fairchildsemi.com/ds/2N/2N5459.pdf) and the BF245A (http://www.fairchildsemi.com/ds/BF%2FBF245A.pdf).

Any ideas how to easily measure it with reasonable accuracy given that it is 'very large'?

Cheers,
Hans.
__________________
In the beginning the Universe was created. This has made a lot of people very angry and been widely regarded as a bad move. --- Douglas Adams
  Reply With Quote
Old 14th September 2006, 02:48 PM   #2
richie00boy is offline richie00boy  United Kingdom
Did it Himself
diyAudio Member
 
richie00boy's Avatar
 
Join Date: Nov 2003
Location: Gloucestershire, England, UK
How high is the input impedance of 'a' jfet transistor?
Look at the gate-source and gate-drain capacitances and work out the impedance at the frequencies of interest.
__________________
www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, modules and more.
  Reply With Quote
Old 14th September 2006, 02:49 PM   #3
jan.didden is offline jan.didden  Europe
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: The great city of Turnhout, BE
Default Re: How high is the input impedance of 'a' jfet transistor?

Quote:
Originally posted by Hans L
What kind of value can I presume a jfet transistor has for its input impedance? Tens of MOhms? GOhms? I've searched datasheets, but apparently the input impedance (which naturally is presumed 'very high' for jfet) isn't stated. Can it be determined from other specs in the datasheet? For instance via the Zero-Gate Voltage Drain Current? I'm currently testing (read: learning) with the 2n5459 (http://www.fairchildsemi.com/ds/2N/2N5459.pdf) and the BF245A (http://www.fairchildsemi.com/ds/BF%2FBF245A.pdf).

Any ideas how to easily measure it with reasonable accuracy given that it is 'very large'?

Cheers,
Hans.
Basically it is so high that the leakage current across the leads over the package dominates any input current at DC. At AC, the input impedance is dominated by the input capacitance, which should be stated in the data sheet. Trying to measure a static input R is probably fruitless.

Jan Didden
__________________
Music is dither to the brain; lets me think below the usual chaos - me
Get more Linear Audio for less! Check out my Autoranger and SilentSwitcher
  Reply With Quote

Reply


How high is the input impedance of 'a' jfet transistor?Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
High input impedance X circuit zelter Pass Labs 4 31st March 2009 10:06 PM
Low amp Impedance input and high dac output tulip Tubes / Valves 1 30th November 2007 08:24 PM
IGC with High input impedance! GregGC Chip Amps 0 30th April 2004 05:03 PM
P-JFET pass transistor borges Solid State 2 14th December 2003 07:01 AM
P3A question: high input impedance sobazz Solid State 1 14th July 2003 05:45 AM


New To Site? Need Help?

All times are GMT. The time now is 02:19 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 16.67%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright ©1999-2017 diyAudio
Wiki