differential input stage

[xuesong]

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.

[djk]

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).

[darkfenriz]

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

[Jef Patat]

ok, what djk said is right.
i just want to make it a little clearer.

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 :

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