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|26th December 2004, 01:13 AM||#1|
Join Date: Jan 2004
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
|26th December 2004, 07:00 AM||#2|
Join Date: Feb 2001
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).
|26th December 2004, 01:38 PM||#3|
Join Date: Jun 2004
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 :
diffy gain = 10
feedback voltage divider =1/20
(music_in - 1/20*music_out ) *10= diffy_out
music_in - 1/20*music_out=1/2000 music_out
this is diffy output
pure mathematics, isn't it?
|27th December 2004, 02:08 PM||#4|
Join Date: Dec 2004
ok, what djk said is right.
i just want to make it a little clearer.
or, to state you :
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
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