These stages primarily may consist of power devices like the power transistors (BJTs) or MOSFETs.
Using power transistors are more common due to the greater ease of configuring them. However when higher outputs are desired, it becomes necessary to add more number of these devices together.
And as per the rules, it becomes necessary to connect them (power transistors) in parallel. Though using transistors in electronic circuits is pretty easy, connecting them in parallel needs some attention due to the one significant drawback with transistor characteristics.
As per transistor specs, the devices needs to be operated under reasonably cooler conditions and that's why we install heatsinks on them to maintain the above criterion. Moreover, transistors have the "bad habit" of conducting proportionately rising currents through them as they get heated up.
Therefore if its case temperature tends to increase, the current through it also increases, which in turn heats it up further. The process gets into a kind of chain reaction heating the device rapidly until the device becomes too hot to sustain and gets permanently damaged. This situation is called thermal runaway, in transistors.
When two or mote transistors are connected in parallel, due to slightly differing individual characteristics, the transistors in the group may dissipate varying amounts of current through them.
Consequently, the transistor which may be conducting slightly more current through it might start getting heated up faster than the neighboring devices and soon we may find the device entering into a thermal runaway situation damaging itself and subsequently causing the phenomenon to the remaining devices as well, in the process.
The situation can be effectively tackled by adding a small value resistor in series with the emitter of each transistor connected in parallel. The resistor inhibits and controls the amount of current passing through the transistors and never allows it to go to dangerous levels.
The value should be appropriately calculated, as per the magnitude of the current passing through them.
How it's connected? See the figure below.
A Simpler Approach
Although the use of emitter current limiter resistors looks good and technically correct, a simpler and a smarter approach could be to mount the BJTs over a common heatsink with a lot of heatsink paste applied to their contact surfaces.
This idea will allow you to get rid of the messy wire-wound emitter resistors.
Mounting over a common heatsink will ensure quick and uniform sharing of heat and eliminating the dreaded thermal runaway situation.
Moreover since the collectors of the transistors are supposed to be in parallel and joined with each other, the use of mica isolators no longer become essential and makes things much convenient as the body of the transistors get connected in parallel through their heatsink metal itself.
It's like benefiting from both ends...transistors combine in parallel through heatsink which in turn eliminates the current hogging situation and gets rid of the bulky emitter resistors.