Hello! This is the second part of the differential amplifier course! If you like electronics and want to know more about how circuits work, check out the full version of the course here:
first of all 15 and -15 voltage do not exist they are 30 and 0 and your gnd is between them at 15 volts , second is the fact that the emitter resistor is common to both transistors as a result whenever you open a transistor more it results in more current over that resistance as such the potential between the base and emitter of the other transistor will decrease closing the transistor which in turn will result in a higher voltage at the collector that s practically how it works , third fact the so called current source it`s just a biased trans which in turn is using the same principle of feedback explained above , whenver the voltage across the emitter resistance of 1k increases results in a lower potential between the base and emitter of that current source transistor therefore a lower bias and transistor is starting to close keeping the current the same but most importantly acting as a variable voltage source (which is what your transistor constant current source is ) that variable voltage has the same behaviour the was previously occupied by that solo resistor, anyways in retrospect a differential amplifier is a constant current source one in which whenever a leg has more current results in closing the opposite transistor via increasing the emitter voltage drop
Look at schematics -+ is the correct way, why do you think they have center tapped transformer and negative voltage regulators. Yes between -15 and +15 is 30 volts of potential difference. The ground and scheme of the circuit is what makes his label correct.
