Help! What would be the input inpedance of this input stage? - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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 10th September 2011, 06:26 PM   #1
shaan is offline shaan  India
diyAudio Member
 
shaan's Avatar
 
Join Date: Mar 2009
Location: Near the Ganges
Default Help! What would be the input inpedance of this input stage?

Picture attached. Symmetrical emitter feedback.

What is the input impedance of this stage at the arrow-marked point?

Thanks in advance.
Attached Images
File Type: gif Input.GIF (4.4 KB, 112 views)
  Reply With Quote
Old 10th September 2011, 06:38 PM   #2
diyAudio Member
 
Join Date: Nov 2005
About 15k? Can't you look at the current of V3?
  Reply With Quote
Old 10th September 2011, 06:40 PM   #3
shaan is offline shaan  India
diyAudio Member
 
shaan's Avatar
 
Join Date: Mar 2009
Location: Near the Ganges
Just tested the current. its 40uA.
  Reply With Quote
Old 10th September 2011, 06:43 PM   #4
shaan is offline shaan  India
diyAudio Member
 
shaan's Avatar
 
Join Date: Mar 2009
Location: Near the Ganges
Quote:
Originally Posted by shaan View Post
Just tested the current. its 40uA.
Opps made a mistake in the measurements. The correct value is 19.5uA @1V peak.

So, it is ~15K?
  Reply With Quote
Old 10th September 2011, 06:53 PM   #5
diyAudio Member
 
Join Date: Nov 2005
40uA is 25k. 19.5u is about 50k at 1V.

15 was just a quick (dumb) guess.

I got 25k when just loading the emitters at 10k, using 3904/06.
  Reply With Quote
Old 10th September 2011, 06:56 PM   #6
shaan is offline shaan  India
diyAudio Member
 
shaan's Avatar
 
Join Date: Mar 2009
Location: Near the Ganges
Hmm... So it's equal to or higher than say, 47k? Safe here?
  Reply With Quote
Old 10th September 2011, 06:58 PM   #7
diyAudio Member
 
Join Date: Nov 2005
If you get 19.5uA with your assumedly more complete circuit, yeah.
  Reply With Quote
Old 10th September 2011, 07:12 PM   #8
shaan is offline shaan  India
diyAudio Member
 
shaan's Avatar
 
Join Date: Mar 2009
Location: Near the Ganges
I see. THANKS A TON FOR THE HELP Andrew!
  Reply With Quote
Old 10th September 2011, 07:48 PM   #9
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
You will get 24k just from the resistors.
  Reply With Quote
Old 10th September 2011, 08:16 PM   #10
sbrads is offline sbrads  United Kingdom
diyAudio Member
 
Join Date: Jun 2005
Location: Kent, UK
Quote:
Originally Posted by DF96 View Post
You will get 24k just from the resistors.
Don't think so. Don't forget there's the same voltage at each outer end of the 12k resistors. Ohms law and all that.

Roughly, I make it the following resistive (mostly) impedances in parallel..........

12k//12k in series with 47k=> 53k
100k//100k assuming low impedance to earth supplies => 50k
i/p Z of 2 transistors in // - not sure here, may be very high due to feedback so I'll ignore it. No, call it 1M.

So, 53k//50k//1M = approx 25k (to an AC signal) (40uA @ 1v)

Last edited by sbrads; 10th September 2011 at 08:26 PM.
  Reply With Quote

Reply


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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Help with input stage driver for EL84 output stage AudioGeek Tubes / Valves 2 20th March 2010 01:38 AM
How to design input stage with specific input impedance/pot value mahleriana Tubes / Valves 0 16th November 2009 09:37 PM
Adding a third input to a differential input stage? maudio Solid State 15 11th October 2006 02:16 AM
current bias j-fet input 2sk170 ,2sj74 at input stage YUTK Solid State 11 2nd June 2005 03:34 AM


New To Site? Need Help?

All times are GMT. The time now is 03:59 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2