Help with designing differential amplifier - 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 23rd February 2009, 01:59 PM   #1
diyAudio Member
 
Join Date: Dec 2008
Unhappy Help with designing differential amplifier

hello..
i am a newbie in the forum.
reasonably at ease with electronics and working hands on..
built a couple of chipamps based on 1875 and 3886.

Now i want to build a SS power amp.
I have read Randy sloanes and douglas self books on the topic

i wanted to design a small/ medium power amplifer myself as i think it ll be better for my own understanding of the subject..

the amp is based on Linn topology and draws heavily from sloanes teaching ( i think it is far easier)

as in Lin topology i wanted to first design the three stages individually.

attached below is my first attempt at designing the input stage

i am facing several problems with the design.

1. The current source is not sinking 1 ma as designed.
2. The mirror is not effective
Initially the sink current starts off from 220 Ua and the current is evenly split btw the mirror arms.
after sometime(abt a min) the sink current reduces to 40 Ua and the current is not matched. the current in the arms differs by 3-4 Ua.
and slowly the sink current reduces to 0.

I am able to understand the working based on tested designs of sloane and debug it ,but am not able to design a simple one myself.

Help please...!!
Attached Images
File Type: jpg diff amp.jpg (22.8 KB, 255 views)
  Reply With Quote
Old 23rd February 2009, 02:14 PM   #2
TzeYang is offline TzeYang  Malaysia
diyAudio Member
 
Join Date: Jun 2006
The Input BJTs must be forward biased at the very least.

If it isnt on, there won't be any currents flowing from C to E.
  Reply With Quote
Old 23rd February 2009, 02:16 PM   #3
diyAudio Member
 
Join Date: Dec 2008
thank you..
i ll amend that and get back..
  Reply With Quote
Old 23rd February 2009, 02:18 PM   #4
diyAudio Member
 
Steve Dunlap's Avatar
 
Join Date: Sep 2008
Location: Here
First, set the diode current at 1mA by changing R2 to 20K. Make sure the diodes are still good. At 1mA they should each have 0.6V across them. Change R1 to something close to 600 ohm.

You must have a DC bias source for both transistors in the diff pair. 47K to ground will give you that bias so you can check operation.
  Reply With Quote
Old 23rd February 2009, 04:35 PM   #5
diyAudio Member
 
Join Date: Dec 2008
@steve dunlap and @ tze yang

thank you..
it did work
i am posting the updated version
the mirror is accurate to the seconf decimal in ma.


sink current is set to , as u can see, about 100ma
thank you again.

Attached Images
File Type: jpg diff amp.jpg (26.6 KB, 194 views)
  Reply With Quote
Old 23rd February 2009, 05:06 PM   #6
diyAudio Member
 
Steve Dunlap's Avatar
 
Join Date: Sep 2008
Location: Here
Quote:
Originally posted by krish2487
@steve dunlap and @ tze yang

thank you..
it did work
i am posting the updated version
the mirror is accurate to the seconf decimal in ma.


sink current is set to , as u can see, about 100ma
thank you again.


It looks more like 60mA. That puts the transistor dissipating 1.2W. You might want to lower that by at least a factor of 10.

R2 is still low. Try setting the diode current at 1mA.
  Reply With Quote
Old 23rd February 2009, 05:11 PM   #7
diyAudio Member
 
Join Date: Dec 2008
well
the sim is actually showing 100 ma..

i then changed R2 to 560Ohms

and likewise with R1


The sink current is now at 1.6 ma and the mirror currents are at abt 775Ua
  Reply With Quote
Old 23rd February 2009, 06:37 PM   #8
diyAudio Member
 
Steve Dunlap's Avatar
 
Join Date: Sep 2008
Location: Here
The sim probably runs at 25C. the diodes will heat up at that current and their voltage drop will decrease. I was just guessing. I didn't sim it.
  Reply With Quote
Old 23rd February 2009, 07:54 PM   #9
CBS240 is offline CBS240  United States
diyAudio Member
 
CBS240's Avatar
 
Join Date: Aug 2005
Location: K-town
Hi

Q1 & Q2 bases need to have a voltage potential of half Vcc or +10V. (GND, 0V if you use +/-10V supply) This is where the input and feedback (-input) nodes will be. If they are tied to the positive supply, there will be no Vce left for Q1 & Q2. Base voltage level can be set with voltage divider bias or GND of a split supply.
BC547 has lots of current gain so base current shouldn't be more than a couple of uA. (BTW C grade is the highest gain class and A grade is the lowest.)

The idea of the mirror is to split the current and keep Ic of both sides equal.

Forward voltage drop of one diode and Vbe of the transistor should be about the same. So 1 forward PN juction voltage (0.6V) is left across R1. Also you can research
2 transistor CCS and self bias J-fet CCS.

Emitter degeneration may be needed for Q1 & Q2 unless you go to specific lengths to match them.....or buy a matched transistor array, probably more expensive. The emitter resistor will make the % difference in the transistor variables less, at the expense of some gain. It's a good trade-off.
__________________
All the trouble I've ever been in started out as fun......
  Reply With Quote
Old 23rd February 2009, 09:56 PM   #10
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
download Leach low tim and read the section detailing the input stage.

Look up the Pass and ESP sites for more info.
__________________
regards Andrew T.
  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
Differential to differential amplifier Cauhtemoc Solid State 3 15th August 2007 07:39 AM
differential amplifier Thomas Giz Digital Source 1 23rd March 2003 08:07 PM


New To Site? Need Help?

All times are GMT. The time now is 01:49 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