Why do we need a positive and a negative supply voltage in a differential amplifier?

What will happen if the two beta values are different in a differential amplifier?

 

Please stay on one thread:

 

Now I am resurrecting the thread to know the answer.

 

The Output signal of the differential cascade can and must be as positive, so and negative. This depends on that, where voltage more. On inverting input (the negative output signal) or noninverting input (the positive signal output signal). For positive output signal necessary positive source. For negative output signal necessary negative source.

 

A difference amplifier can be single or dual supply.