Need help understanding current sourced common emitter/source amplifiers - 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 25th February 2011, 01:41 AM   #1
GregH2 is offline GregH2  Australia
diyAudio Member
 
GregH2's Avatar
 
Join Date: Oct 2009
Default Need help understanding current sourced common emitter/source amplifiers

Hello all,

I'm still having trouble grasping a basic amplifier concept I'm hoping you can help with. I'm thinking out loud here so bear with the lecture.

With a common emitter/source amplifier, as most of you will know, it is common to add an emitter resistor (Re) and collector Resistor (Rc) to stabilise the gain and make gain more or less independent of transistor variations. So gain essentially becomes Rc/Re (within reasonable limits).

So we now have a circuit that can be used in multiple amplifiers and perform in much the same way despite transistor variations.

One way to improve linearity and reduce distortion is to replace Rc with a constant current source, which if well designed has essentially infinite output impedance.

This means that gain theoretically becomes infinity/Re, but obviously gets limited by the transistor transconductance and other transistor properties.

So my question is, how do you make a simple current sourced common emitter amplifier operate independently of transistor variations?

I need to make two channels of an amplifier, and each has a current sourced common source jfet input. How do I make both input stages have the same gain without either removing the current source or resorting to a LTP input?

Any advice you can provide will be appreciated!

Regards,

Greg.
  Reply With Quote
Old 25th February 2011, 04:58 AM   #2
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Quote:
Originally Posted by swordfishy View Post
This means that gain theoretically becomes infinity/Re, but obviously gets limited by the transistor transconductance and other transistor properties.
The voltage gain is:

Av= GmRl

Where Rl= 1/Hoe || Rc || Ri (next stage/load resistance)

1/Hoe being the inherent collector dynamic resistance. You can usually ignore it with minimal errors since Rc is usually ~an order of magnitude smaller. An active load can increase gain by making:

Av=~ (1/Hoe)Gm (no load)

Quote:
I need to make two channels of an amplifier, and each has a current sourced common source jfet input. How do I make both input stages have the same gain without either removing the current source or resorting to a LTP input?
I wouldn't worry about it. You could always match transistors, but for the most part, the gains won't be that far off for any reasonable design. How much gain do you really need here anyway? How accurate do you need to be?
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 25th February 2011, 08:46 AM   #3
diyAudio Member
 
Join Date: Jun 2005
Location: Zürich
Active load techniques are not particularly useful without feedback for the reasons you mention. They are mostly seen within opamp topologies where global feedback stabilizes the operating point.

Samuel
  Reply With Quote
Old 25th February 2011, 10:52 AM   #4
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
the active collector load will almost certainly not have constant impedance across the whole passband of the amplifier. This results in gain that varies with frequency and confirms SG's need to apply NFB.
JLH shows a variety of one, two and three transistor amplifiers with different ways of providing AC feedback and DC feedback. All these circuits appear individually in other papers, so far I have only seen the collection of them in "The Art of Linear Electronics"

Where else can we find a comprehensive collection of feedback scenarios for the one to three transistor amplifiers?
__________________
regards Andrew T.
  Reply With Quote
Old 25th February 2011, 12:11 PM   #5
GregH2 is offline GregH2  Australia
diyAudio Member
 
GregH2's Avatar
 
Join Date: Oct 2009
OK thanks, I will get JLH's book and look into local NFB options. In the mean time I'm going to play with some simulations and build some test circuits.

Greg.
  Reply With Quote
Old 25th February 2011, 12:17 PM   #6
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
JLH's The Art of Linear Electronics is out of print.
It was published by Newnes isbn 0-7506-3746-3

I consider this my best electronics reference book.
Better than Horowitz and Hill and the ARRL handbook.
It is a good complement to Self and/or Cordell.
Those 5books + a number of briefer pamphlets & small paperbacks cover most topic I need to consult.
__________________
regards Andrew T.
  Reply With Quote
Old 25th February 2011, 08:59 PM   #7
diyAudio Member
 
Join Date: Apr 2010
Location: Coffs Harbour
Chapter 3 of Self's recent title "Small Signal Audio Design" is all about discrete amplifier circuits in the standard categories of single, 2 and 3 transistor amplifiers and opamps. It's yet another book but this one is right in the audio ballpark with up to date analyses.

As a comment, the book is light on theory whilst retaining essentials and thorough on evaluation and choices as would be necessary for any DIYer to make. BJT in this chapter only.

The publishing details and a little more on discretes are on The Douglas Self Site
__________________
Ian
  Reply With Quote
Old 26th February 2011, 02:59 AM   #8
GregH2 is offline GregH2  Australia
diyAudio Member
 
GregH2's Avatar
 
Join Date: Oct 2009
OK thanks all, I will track down those books. Ordered the JLH one already.

Greg.
  Reply With Quote
Old 26th February 2011, 03:02 AM   #9
GregH2 is offline GregH2  Australia
diyAudio Member
 
GregH2's Avatar
 
Join Date: Oct 2009
Thanks Miles,


Quote:
Originally Posted by Miles Prower View Post
The voltage gain is:

I wouldn't worry about it. You could always match transistors, but for the most part, the gains won't be that far off for any reasonable design. How much gain do you really need here anyway? How accurate do you need to be?

Well I don't know really. I just assumed they would need to be pretty close if they're for the L and R channel of the same amp. So you think I'm worrying too much then and that I should just build it as is and match Vgs as best I can?
  Reply With Quote
Old 26th February 2011, 06:14 AM   #10
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
^
|
|

Yes.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  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
Common Source versus Common Drain output stages alaskanaudio Solid State 36 1st June 2014 02:35 AM
Basic Common Emitter Amp Help Obe1 Analog Line Level 99 21st February 2010 05:08 PM
Current Sourced Cathode Follower dr._sleep Tubes / Valves 8 27th February 2006 08:46 PM
Current sourced heaters in a CF Salas Tubes / Valves 21 31st May 2005 09:35 PM
Current sourced heaters Salas Tubes / Valves 3 26th January 2004 11:09 PM


New To Site? Need Help?

All times are GMT. The time now is 05:30 PM.


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