output stage & driver stage-Audio Amplifiers

[fantabulous68]

I want to know what happens if the output transistors were to heat up, what would happen to the quiescent current in the output stage if the VBB value was kept constant?

All i know is that Transistors have a negative temperatuere co-efficient. If the o/p transistors get hot then they need less drive and this causes thermal runaway; but what would happen to the quiescent current in the output stage if the VBB value was kept constant?

Thermal runaway occurs in Power BJTs that are subject to heating in constant VBE bias conditions. As devices heat up (for a constant collector current)
-->VBE decreases by 2mv/degree celcius
--->under constant Vbe the collector current increases

this leads to more self heating and further increase in collector current, usually to destruction


a VBE multiplier circuit compensate for this effect?right?
if the transistor of the vbe multiplier is kept in close thermal contact with the output devices then as these devices heat up
all their vbe's will reduce similarly (by 2mv/°C) and hence the bias level effectively remains constant. Thus the vbe multiplier compensates very well for output device heating effects and eliminates possible thermal runaway.

[Audioguru]

Two of us already answered these simple questions hours ago on another website.

Please do not duplicate your questions all over the place.