Electronics > Beginners
My first audio amplifier (and it's really bad)
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jtruc34:
I'm requesting your help because I can't make it work very good. By very good, I'm mean relatively to what I expected from it, and by that I mean very poor quality but at least the sound gets louder without saturating at Vcc/5.

Attached below is the schematic of it.

As you can see, it works with a general purpose LM358P OpAmp (I don't expect it to be very great for audio), followed by an emitter follower based on a Darlington pair of two random BJTs I had on my bench.

I won't discuss about the quality of it, the only big problem is that it saturates like crazy when the output is more than 2V peak-to-peak and I'm not quite sure why (it's a way to say that I haven't got any idea).

The strange thing is that if I increase Vcc, it ceases to saturate for one second and then it saturates again. It's like the operating point of the transistor changes for a second while the capacitor charges or discharges itself.

I tried several things. I'm not sure about what value to put for Re. All I've noticed is that under 100 Ohms, no significant improvement can be observed apart from the temperature of the room, and above 10 KOhms, the output signal amplitude decreases too much.

The value of C doesn't seem to change anything, except that under 100nF, the low frequency are cut off too much.

The whole point of the capacitor was to avoid biasing the transistor myself, but since its emitter is connected to -Vcc (in the beginning, it was connected to 0V), I'm not sure wether it is useful anymore. I tried to connect the loud speaker directly to the emitter of the transistor without any major improvement, or without any improvement at all.

I tried to connect the other pin of the loudspeaker to -Vcc (it is a very cheap loudspeaker, under $1, I don't really mind if I destroy it accidentally), and it improved things a bit, but that's a very ugly "solution" since it can damage the loudspeaker. Actually, I don't really know why this improved the saturation.

So, two questions : how do I reduce the saturation, and how do I choose Re and C?
MagicSmoker:
Erf... Re is typically a small value to provide a bit of local negative feedback (called "emitter degeneration"). You also need to insert a resistor in series with the collector and take your output from the collector.

Finally, you'll also need to modify the circuit so that the transistors are biased into conduction with a current through the collector load resistor equal to the maximum current you wish to draw from the amplifier (Class A operation). Applying a constant positive voltage to the op-amp non-inverting input will get the ball rolling, but this will NOT be stable. I leave it to you to dig a little deeper into how to solve this problem (hint - diodes can help here).

HB9EVI:
also bare in mind, that the LM358/324 is not a good choice for Audio; it's an opamp for DC which can go close to negative/gnd rail, what makes it a good single supply opamp, but not for AC because of the crossover distortion.
Zero999:
The loop isn't closed at DC, so it won't work. The output will just clip at either extreme.

Yes, people may complain about the LM358 having crossover distortion, but that's a non-issue for this circuit because it only sources, never sinks current, so no crossover distortion.
mikerj:
Don't vary the closed loop gain to change the output as you are currently doing.  Fix the closed loop gain, and use the potentiometer to vary the input into the amplifier.  You will need to bias the input at around half the supply voltage.

What is the value of Rc and what is you speaker impedance? As someone else has mentioned this design is (or will be) a Class A amplifier since your output stage can only source current, you have to rely on Rc to sink it.  With a suitable value of Rc, and correct biasing this can work but will be horribly inefficient, the output transistor and Rc will be burning lots of power even with little or no output, requiring a big heatsink. 

Consider adding a complementary PNP output stage so that your amplifier can "push" and "pull" current, you will then have a Class B output.  It will sound bad with lots of crossover distortion, so then you can find out how to bias both output transistors into partial conduction to fix this, and end up with a Class AB, which is how the vast majority of linear amplifiers operate.

The 10uF output capacitor is woefully undersized for a typical 4 or 8 ohm speaker, this will block all low to mid frequencies; for an 8 ohm speaker it will start rolling off below about 4kHz, (8kHz for a 4 ohm speaker!).  Even for low fidelity you need at least 10 times the capacitance, and it you want to put music through it maybe 100 times.
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