Electronics > Beginners
My first audio amplifier (and it's really bad)
jtruc34:
Until now, the signal was coming from my function gen. I just output a sine wave between 50 and 100 mVpp, and adjusted the gain until I saw saturation.
So, I will try your modification as soon as I can and will say if it works better.
But why do Re have to be so small ? In my (very small) experience, it works even with bigger values.
And finally, so, what was the reason the output saturated only at about 2Vpp when then rail is at 5 times the voltage? From what I read of what you wrote, is that because my configuration amplified the offset of the Opamp? I'm not quite sure since the capacitor was precisely removing this DC component. Also, I'm not quite sure why the feedback is connected to -Vcc and not to ground through the divider.
Thank you for your help.
EDIT:
--- Quote from: MagicSmoker on May 10, 2019, 11:39:15 am ---EDIT - well I see that Zero999 handed you the solution on a silver platter! :-DD
--- End quote ---
Yes, indeed, and that's very kind of him. In general, I see that people on this forum are very kind and helpful. By the way, I should emphasise that my point is more to understand why it works or doesn't work than to make it work and use it, because, as everyone said, it will be awfully inefficient anyway.
Zero999:
--- Quote from: jtruc34 on May 10, 2019, 11:49:16 am ---Until now, the signal was coming from my function gen. I just output a sine wave between 50 and 100 mVpp, and adjusted the gain until I saw saturation.
So, I will try your modification as soon as I can and will say if it works better.
But why do Re have to be so small ? In my (very small) experience, it works even with bigger values.
And finally, so, what was the reason the output saturated only at about 2Vpp when then rail is at 5 times the voltage? From what I read of what you wrote, is that because my configuration amplified the offset of the Opamp? I'm not quite sure since the capacitor was precisely removing this DC component. Also, I'm not quite sure why the feedback is connected to -Vcc and not to ground through the divider.
--- End quote ---
Re forms a potential divider with the output load. The transistor can only pull the voltage across the load, towards the positive rail. It's the job of Re to pull the load voltage towards negative. Lowering Re will increase the possible negative output swing, at the expense of greater power dissipation. Increasing it will have the reverse effect. If you don't have adequate heatsinking, then no doubt it will work better, if Re is high, but you won't get so much voltage swing.
Re could be replaced with a constant current sink, but that would require another transistor, which would be better employed converting it to class AB operation.
The feedback potential divider is AC coupled to -V and has a DC gain of 1. It theoretically makes no difference whether it goes to 0V, -V or +V, since it has the same effect at AC, as the power supply rails should be properly decoupled i.e. have capacitors in parallel with them. The reason was to provide DC bias across the coupling capacitor.
jtruc34:
OK, thank you very much. But since my loud speaker has about 150 Ohms impedance, Re probably doesn't need to be that low, does it?
And I still don't understand why my amplifier was saturating at only Vcc/5 output.
Zero999:
--- Quote from: jtruc34 on May 10, 2019, 12:40:05 pm ---OK, thank you very much. But since my loud speaker has about 150 Ohms impedance, Re probably doesn't need to be that low, does it?
And I still don't understand why my amplifier was saturating at only Vcc/5 output.
--- End quote ---
You should have stated the impedance of the speaker, otherwise people assume 8Ohms which is the most common one. I agree, for 150Ohms, Re can be higher, try 100R to start with.
The output was saturating because Re drops a significant voltage.
jtruc34:
I already stated it in another answer, according to what someone else asked me, sorry that you didn't see it. Yes, 100R is about what I have.
So, if it is because of Re, changing its value would have made things better, but it didn't... I'm lost |O :-DD
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