Need help with a 3 stage amplifier

I am working on my final project for class and am STUCK! I am to design a 3 stage circuit with stage 1 and 2 using a 2N3904 transistor in a votlage divider config. and stage 3 using a darlington transistor in a voltage divider configuration leading to an 8 ohm speaker. I have stage 3 of the configuration done and I get the correct wattage out of the 8ohm speaker. But for the life of me I cant figure out how to determine the values for the resistors in stage 1 and 2 to give me the correct Vb in stage 3. Where do I start these calculations?

• Is it the case that you expect $R_4$ to be a speaker? Your final stage is simply "not done" except in "extreme youtube" videos. The reason is that for half the drive cycle, $R_1$ has to supply needed current. That 3rd stage needs to be changed. Is it required, as shown? Also, you don't have any global NFB. And given your topologies using $C_5$ and $C_3$, you will almost certainly require some. – jonk Aug 2 '18 at 18:59
• Yes that is exactly how it is supposed to be per the guidelines. I was able to calculate stage 3 and get the correct power output at R4. I just need to figure out how ot get the correct voltage into the 3rd stage. – raven Aug 2 '18 at 19:04
• In emitter followers, the DC current must always exceed the AC load current which is why this config is not wise to drive a speaker. I.e. R1<= R4 – Sunnyskyguy EE75 Aug 2 '18 at 19:04
• Yeah I dont doubt that it isnt the best choice. I think the point of having to create this circuit is to use engineering problem solving skills...right now im exercising my "look to those with more knowledge" skills haha – raven Aug 2 '18 at 19:07
• Start at the end, then. Work out the details of stage 3. (It will be ugly.) It's clearer now why you were told to use a Darlington. – jonk Aug 2 '18 at 19:24

The resistors in stages 1 and 2 don't set $V_{B}$ for the third state...R2 and R3 do that. The stages are capacitively coupled so the biasing resistors in each stage set the transistor bias point for that stage.