差模与共模

snowes7

差模与共模
小弟一直不太明白差模与共模，差模增益是对两端输出的差进行放大吗，然而两端输出的信号不是方向相反、大小相等的吗？他们的差显然是原单端信号的二倍吗，那不是本身就放大了两倍吗？这样想对吗？
而共模增益是对单端说的吗？（因为不是两端输出的信号相同吗，我们把它看成一端）
小弟迷惑中！！！！！！！！！！！！！！！！！！！

niklaszhu

sorry, i have to  type English
normally, amplifier works in differential input and differential output
assume two differential inputs are Vin1 and Vin2, differential output are Vout1 and Vout2, so differential gain is (Vout1-Vout2)/(Vin1-Vin2)
for common mode, two input have same voltage, Vin, and the output differential voltage( Vout1 - Vout2) should be zero in ideal case.
however, any circuit cannot be such symmetry, so common gain is (Vout1-Vou2)/Vin
in principle, we always use "common mode attenuation" for AC characteritics, the difinition is differential gain divide common gain

snowes7

多谢了！！！！！！我还想问一下，差动对电路的目的是什么啊？我们要它的Vout1 - Vout2用于后续电路，还是用某一端的输出，各有什么意义！！！！！！！！期待您的回复！！！！！！！！！！！！！！！！！！！！！！！！！

niklaszhu

differential input can attenuate the common mode noise, for example, Vin1' = Vin1 + Vnoise, Vin2' = Vin2 + Vnoise, then Vin1'-Vin2' = Vin1 - Vin2, so it cancel the noise
whether using differential output or single ended ouput, it depends on the following stage of your circuit. If the next stage is single input, so use only one output, if not, use two outputs

snowes7

那差分输出是没噪声的，而单端输出还是有噪声的阿？？？？？？？？？？

niklaszhu

you are confused about the noise.
output noise can be divided into two parts: input noise and system itself have some kind of noise.
differential input can attenuate common mode input noise,it says attenuate input noise.
so, both differential output and single ended output have noise, because of the system itself, however, input noise is attenuated in both cases.

snowes7

哦！我明白了，多谢仁兄！！！！！！
我想问一下这位仁兄是做什么的阿？

careking

强

niklaszhu

RFIC, sigma-delta frequency synthesizer design

snowes7

我是想致力于模拟集成电路设计的本科生，即将成为一名硕士，我的研究方向是光纤通信用的ic设计，小弟初来乍到，希望多多指点！我的qq475548502,多多交流！！！！！！