I am trying to build a simple amplifier to drive an 8ohm, 0.7W speaker using a 5V single supply. Audio fidelity is of little importance, since it is a cheap speaker, but I would like the output to be recognisable at the very least. I built the following circuit based on designs I've found on the internet but it doesn't work.
simulate this circuit – Schematic created using CircuitLab
I have checked it with my scope and it seems to work fine everywhere except at the transistor emitter. I suspect that the problem is caused by C2 or the gain resistors, R4 and R5, as removing them, so it is in unity gain configuration, gives an output, albeit with a DC bias.
I would greatly appreciate any suggestions for improvements on this circuit, or different circuits of similar complexity.
Thanks in advance 🙂
EDIT: I went with Andy aka's answer because it ticked the boxes of simplicity and I was able to build it with parts I had on hand. I combined it with my original circuit, so essentially the only modification I made was the addition of the PNP transistor. I changed the values of the feedback resistors to be equal, giving a gain of 2 as I got significant clipping for anything above this value. Overall the audio quality is reasonable, especially since the speaker I am driving is quite low quality and is probably going to be a limiting factor for audio fidelity.
Thank you to everyone who answered
Best Answer
Consider a simple push pull stage like this one that I have modified from a picture on the internet (but take note of stuff I've said further down): -
There will be some cross-over distortion but, because the output transistors are "within" the feedback loop of the op-amp it won't be too annoying.
However, take note that the LM358 is really poor at delivering a high level output voltage on a restricted supply voltage - I would definitely look for an op-amp that has a rail-to rail output.
If you got a R2R op-amp, the output voltage to the speaker would be about 3.5 Vp-p (sine wave maximum) and this is an RMS of 1.24 volts hence, the power into an 8 ohm speaker would be limited to about 200 mW with some distortion.
However, if it were a square wave fed to the speaker, the power out would be about 380 mW.
If you definitely need to get close to 0.7 watts then consider using an output transformer or a bridge amplifier. The problem with a single push-pull stage is delivering the peak-to-peak voltage from a 5 volt limited supply.