differential 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 26th December 2004, 01:13 AM   #1
xuesong is offline xuesong  United States
diyAudio Member
 
Join Date: Jan 2004
Location: atlanta
Default differential input stage

I confuse the principle of a typical differential amplifier for a audio amplifier input stage circuit. I know that a differential amplifer only amplify the difference of two signals, if the two signals input from Q1 and Q2's base (Q1 and Q2 are two transistors of a differential amplifer). I saw, in audio amplifier circuit, one input signal (IN1) is from music source to Q1 base, and another (IN2) is from output as a feedback signal to Q2 base. that means, the differential amplifer only amplify IN1-IN2 and output it from the collector of Q1 to VAS. My question is that : if this is a amplifier without any distortion and IN1=IN2. the difference of two inputs should be zero and with no any signal output. or I will see, actually, the difference of two inputs should be the signal we don't want. I can't figure out how the differential amplifier works in audio amplifer to amplify a music signal after I study all the principle of it. I know there is something wrong with me. Please help me on it

Thanks,

Xuesong.
  Reply With Quote
Old 26th December 2004, 07:00 AM   #2
djk is offline djk
diyAudio Member
 
djk's Avatar
 
Join Date: Feb 2001
Location: USA
If IN1 is higher than IN2, the difference will cause the output of the amplifier to increase untill the signals are the same.

IN2 usually receives only 1/20 of the output signal, so when the difference between IN1 and IN2 is zero the amplifier has 20X voltage gain (26dB).
  Reply With Quote
Old 26th December 2004, 01:38 PM   #3
diyAudio Member
 
darkfenriz's Avatar
 
Join Date: Jun 2004
Location: Warsaw
the fact is a differential stage DOES amplify the common signal and if in1=in2 you will get little fraction of it.
the parameter called CMMR (common mode ?something? ratio) should be as big as possible then...

but aside this

try to imagine :
vas gain=200
diffy gain = 10
feedback voltage divider =1/20
OK.
first stage:
(music_in - 1/20*music_out ) *10= diffy_out
right?

second stage:
diffy_out*200=music_out

so:
music_in - 1/20*music_out=1/2000 music_out
this is diffy output

and:
music_in=(1/20+1/2000)*music_out

pure mathematics, isn't it?
cheers
  Reply With Quote
Old 27th December 2004, 02:08 PM   #4
diyAudio Member
 
Join Date: Dec 2004
Location: gent
ok, what djk said is right.
i just want to make it a little clearer.
Quote:
If IN1 is higher than IN2, the difference will cause the output of the amplifier to increase untill the signals are the same.
this is perfectly true, but i might add that this is maybe to you a common signal, since both IN1 and IN2 have now the same value. In fact this is correct. Only, when for some reason IN2 would change a little, because of time dependent reasons, or very small instabilities this difference will be amplified as a differential input and thus make sure IN1=IN2 very fast.
or, to state you :
Quote:
if this is a amplifier without any distortion and IN1=IN2. the difference of two inputs should be zero and with no any signal output
this is true as well, but with no signal output IN1 won't equal IN2, since IN2 would be zero and thus IN2 would rise again.
In fact this is what will happen. If IN2 equals IN1 the output will drop, causing IN2 to drop, causing differential amplification, causing IN1=IN2 , causing output drop, ...
The diff amp will stabilize itself.

what darkfenriz said about CMMR, common mode rejection ratio, is true as well, but should be of great concern to start off.
I started to build a site as a resource for newbies, maybe you should take a look.

kind regards jef
__________________
Visit Jef's Palace at http://www.jefspalace.be to comment my personal project
  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
JLH with a differential input stage mohan72 Solid State 46 10th September 2008 07:59 PM
Differential I/V stage? And RX input guzzler Digital Source 29 19th December 2006 05:32 PM
Adding a third input to a differential input stage? maudio Solid State 15 11th October 2006 02:16 AM
Differential input stage Dynsdale Tubes / Valves 8 3rd September 2005 07:29 PM


New To Site? Need Help?

All times are GMT. The time now is 07:15 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