hi guys! can anyone explain why Q3 in turned on in this circuit? i could see it when Q2 is turned on and therefore Q3 is turned off (there's no enough voltage between base and emitter of Q3), but i cant figure out how can Q3 be on when Q2 is off, with Q2 off Q4 is off too, so Q3 emitter is not connected to anything given the fact that Q4 collector cant reach ground, it is floating, what im missing?

 

If Q2 is OFF R2 provide a base current for Q3 transistor.
So when you connect the load Q3 will immediately start to conduct the current.

 

so Q3 will conduct (will short output with Vcc) only if an output load is connected? right? i look at it like Q3 is off but when a load in conected to the output it turns on

 

so Q3 will conduct (will short output with Vcc) only if an output load is connected? right?

Yes. But the output will not be short to Vcc. We have a Q3 Vbe voltage drop and D2 diode drop.
Vo_max = Vcc - 2Vbe = 5V - 1.4V = 3.6V (with no load). So when you connect the load Vo will drop.

 

yes, i see it clear now, thanks very much jony for your response! i really appreciated it