Balanced Operation

[gchrist]

Dear fellows,

I am reading the Balanced line stage article from Pass but I have this question to make:

The circuit does not amplify any portion of the signal which is the same at both inputs. It will only amlify the difference of the two imputs.

Question: The two inputs are with opposite phase. Will the difference between them always be zero? Then what does the circuit amplify?

Thanks
George

 

[Holger Barske]

Not the difference is zero, the sum is
The difference ist what is amplified.
If you would simply subtract both inputs, you would get a result wich has double amplitude of one of the both inputs.