Electronics > Beginners
why is class A audio amp saturating?
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d4n13l:
Hi,

Before you say it, I know class A audio amplifier are not efficient, I just decided to make the circuit to see how they work, I want to test the different kinds. Also I'm learning about how op amps work.

I made the circuirt you see in the attrachment and I'm using a small radio as input, however for some reason the output is saturated on the positive side of the wave and can't figure out why. I can hear a clean audio from the speaker but the volume is also low.

Any help will be appreciated.
Benta:
First, calling your circuit a "class A" amplifier is a bit over the top.

Bur to your circuit: your resistances are waaay too high for en LM358. Divide the values by 20 and you'll probably get an acceptable result.

magic:
Surely class A :)

That being said, running constant current through the speaker is undesirable. Firstly, it dissipates a few watts of constant power in the speaker, make sure not to exceed its power rating if it's small. Secondly, it pushes it away from its neutral position, presumably increasing distortion.

As for the limited output - does the opamp run on 9V too? LM358 is limited in output to some 1.5~2V below its positive supply, add 0.7V difference between Q1 base and emitter and you get hard clipping at 6.6V.
mikerj:
Are you powering this with a little 9v battery or a decent power supply?  The LM358 output will likely only swing to about 1.5v below the rail, plus the Vbe drop from the transistor so you are already down to about 6.9v maximum at the speaker.  If the power rail is sagging (e.g. from a small battery, long cables etc.) then it's not a stretch to think you'll only see the 5.6v pkpk that you are getting.  Use your scope to look at the op-amp output and the supply rail. 

Benta's comments about the bias resistor values is also worth heeding, the LM358 is an older bipolar op-amp so input bias currents are something you have to consider.  With no audio input what is the voltage at speaker?  You want it to be at half the rail voltage, but chances are it won't be.

Finally you might know this already, but speakers do not like having large DC biases applied to them like this circuit does.  It causes high power dissipation in the voice coil and biases the cone away from it's mid-point which will introduction distortion. 
d4n13l:
I'm using a 0.5W speaker which should be fine here I think. I'm using a power supply, not batteries, and I'm also feeding 9V to the op amp.

I didn't know the op amp can't go all the way to the supplied voltage, so I guess that's the problem here.

I checked the voltage without input and it is higher than it should at ~5.8V. So I take it this shift from mid-point is caused mostly by the DC bias in the speaker? But that's the problem with this class of amplifier right? the speaker is receiving DC all the time, nothing to do about it. I want to try a push-pull next
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