# 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, 2018 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. Aug 2, 2018 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 Aug 2, 2018 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 Aug 2, 2018 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, 2018 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.