To prevent thermal runaway -
In place of R5 try a low-power npn transistor like a BC109 with collector coupled back to its own base via a slider on a 1K-ohm preset potentiometer. Thermally couple this transistor to the output transistors if possible.
What help? you have no feedback! I suggest you have a look at some amp designs to see how it's usually done.
Looks from here like the circuit is already pretty well designed.
Thermal runaway is avoided by adding appropriate heat sinks. Simple. The resistor values are a calculation that I'm sure your teacher wants YOU to do, and you haven't provided enough information in your post for us to do it for you anyway. We'd need to know stuff like what the gain of each stage should be (we already know the gain of the last stage is 1...), whether the first two stages use matched pairs in a single package or discreet components, etc...
Really, if your teacher didn't think you had the necessary tools (math) to do this, he wouldn't have assigned it.
used
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etc... ^^^^^^^^
Actually, their components have a reputation for being terrible busybodies....
Cheers! Rich
Lol. Was that supposed to be discrete? Got past my language filter it did. :)
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