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previous: R.G. >> ... how when you have two parall... -- 8/2/1997 8:59 AM View Thread

Re: Differential amplifiers?

8/4/1997 3:39 PM
Erik M. Wood
Re: Differential amplifiers?
Ok, that all makes more sense, now. My next curiosity is with the common mode rejection of this circuit using balanced inputs. Assuming the cathode signals were identical but of opposite phase, they would cancel out at their mutual cathode connection to B- except for the common mode noise and RF picked up through the mic cable, say. This stays as a minor voltage fluctuation on the cathodes, in phase with the signals on the grids. Other than this the B- varies very little. Ok so far? Next, because of what you said about the rising cathode potential looking to the circuit like a sinking grid potential, and the same but opposite for a sinking cathode potential, the common mode noise on the cathode would then cancel out the common mode noise on the grids, since the common mode peaks would effect a rise in cathode potential in time with the common mode noise causing an equal rise in grid potential, effectively creating the difference between two out of phase but identical noise signals. In other words, the grids would be going positive while the cathodes would be pulling them back down an equal amount, correct? This would leave nothing but the intended music signal -equal, but still balanced (out of phase)- at the output. Is this right, or is the noise cancellation achieved in another part of the circuit (e.g. the output)? I have had it explained to me that it is not necessary to flip the two signals back into phase in order for this particular design to offer Common Mode rejection. I'm just trying to understand how this works.  
I hope this is clearer than it looks to me.  

R.G. For common mode rejection: Assuming... -- 8/4/1997 5:26 PM