Electronics > Beginners
My first audio amplifier (and it's really bad)
<< < (4/4)
MagicSmoker:

--- Quote from: jtruc34 on May 10, 2019, 03:33:53 pm ---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

--- End quote ---

The feedback resistance ratio given by Zero999 above might be asking too much of a sloppy op-amp like the LM358. As a first-pass I'd limit the voltage gain to around 10x and avoid the use of very large resistance values in general. Try 100k for the feedback resistor and 10k for the shunt resistor.

As for the value of the emitter resistor, that really depends on how much (reasonably linear) negative output swing you need. Consider if it was 100R and the speaker impedance is 100R, then the positive half of the waveform can swing nearly to the positive supply on account of the transistor providing it, but for the negative half of the waveform the best the transistor can do is turn completely off, allowing all of the output to come from the emitter resistor, which being equal to the speaker impedance in this example means you get a maximum of 1/2 the negative supply. This will make for a most unpleasant listening experience even if high fidelity, per se, isn't the goal. With a 10R emitter resistor the negative swing can approach 90% of the rail but standing current increases to around 1A (a 0V output the effective collector-emitter resistance of the transistor will be 10R).

Hopefully that is enough of an explanation to set you on the right path without doing all the thinking for you.

Zero999:
Yes, the LM358 doesn't have sufficient bandwidth for a gain of 200, at audio frequencies, perhaps it will be fine for voice, but not much else. It would be better to split the gain between two stages.


--- Quote from: jtruc34 on May 10, 2019, 03:33:53 pm ---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

--- End quote ---
Forgive me for not reading everything in the minutest detail. In future please put important details like that either in the first post or even better, the schematic. Few people will have time to read everything.

As mentioned above the Re drops voltage. The lower its value, the less voltage it will drop, for a given current, as per Ohm's law. Unfortunately, because the DC voltage across it is 10V, irrespective of its value, the DC current, will also increase, as the value is reduced.

Here's a simulation showing a 100R load with the amplifier set for a gain of 101. The clipping doesn't go away until R3 is lowered to 20R. Increasing it only makes it worse.
Navigation
Message Index
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod