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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.

schematic

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

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  • \$\begingroup\$ You need a DC path from Q1 emitter to Ground, otherwise, there is nothing to pull the output low. \$\endgroup\$ – Peter Bennett Feb 8 '18 at 2:37
  • \$\begingroup\$ It looks to me like there's no way for the capacitor to discharge. You may need a push pull amplifier for the output \$\endgroup\$ – C_Elegans Feb 8 '18 at 2:39
  • \$\begingroup\$ So a resistor in parallel with the speaker and C2? I tried that with a few values, approx 1k to 10k, but it gave a heavily distorted output. \$\endgroup\$ – spacenut1 Feb 8 '18 at 2:39
  • \$\begingroup\$ The output ceiling of the LM358 will be \$V_{DD}-1.5\text{ V}\$, so it will go between 0 and 3.5 V, and the BJT in follower configuration will drop 0.7 V. So the voltage across the SPKR1 will be in the range from 0 to 2.8 V (losing 2.2 V.. ouch). Also C2 has no return path for AC. \$\endgroup\$ – Harry Svensson Feb 8 '18 at 2:39
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    \$\begingroup\$ @spacenut1 Do you want to use an IC (you already are, it appears?) Because, if so, the TDA8551 is the perfect fit for the 5 V power supply. It includes a built in volume control, too. Cheap, easy to use. Otherwise, are you more interested in coming up with a design of your own? \$\endgroup\$ – jonk Feb 8 '18 at 2:42
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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): -

enter image description here

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.

I am trying to build a simple amplifier to drive an 8ohm, 0.7W speaker using a 5V single supply.

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.

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I'd suggest a small class-d amp which are available cheaply these days.

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  • \$\begingroup\$ Agree. 5V is not easy to get it working with using transistor to drive. \$\endgroup\$ – Jason Han Feb 9 '18 at 0:55
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Per Peter Bennett's comment you need a load resistors from the emitter to ground, or there is no way for the voltage to swing from rail to rail. At 5 volts you could use a 5 ohm 5 watt resistor. The speaker 'sees' only the audio signal that passes through C2.

Because of your bias circuit, Q1's emitter is going to be at 1/2 of 5 volts, or 2.5 volts, so you essentially have made a class A amplifier.

2.5 volts across a 5 ohm load equals 1/2 amp of idle current, or 2.5 watts of power as heat. Class A amplifiers have limited power as they do run hot, but the sound is very clean. Q1 will get hot so be sure to mount it to a large heat sink.

HINT: Replace the 5 ohm resistor with a 5 amp 2 mH inductor and you will get more sound, much like the Delco car radios of the 1960's and 70's. The inductor resonates allowing a wider voltage swing.

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    \$\begingroup\$ Thanks for the answer. Unfortunately 2.5W lost across the load resistor to drive a 0.7W speaker seems a bit wasteful. Would this problem be circumvented using a push-pull topology? \$\endgroup\$ – spacenut1 Feb 8 '18 at 2:56
  • \$\begingroup\$ Of course a class AB amplifier is more efficient and the most common amplifier design. Search for 'audio amplifiers' on the web and you will get drowned in a sea of diagrams. Most all of them are class AB with push-pull outputs. \$\endgroup\$ – Sparky256 Feb 8 '18 at 3:01
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If you have any 5V logic inverters try this.

schematic

simulate this circuit – Schematic created using CircuitLab

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You just need an output driver for the LM358. The single BJT won't cut it. Since you have an oscilloscope, there are some important good tests you can make, too. Nice.

Here's a schematic to try with your LM358. Keep in mind that you may need to add a good bypass capacitor across the collectors of the two output BJTs and the supply rails for the LM358 (not shown on the schematic below.) I don't think BJT or LM358 dissipation will be a problem, since the entire amplifier can't drive more than perhaps \$100\:\text{mW}\$ into an \$8\:\Omega\$ speaker. Maybe.

schematic

simulate this circuit – Schematic created using CircuitLab

Ground the input for a few adjustments.

The goal for \$R_6\$ is to have about \$6\:\text{mA}\$ through it. That's also not critical. You could go as low as half that and as high as perhaps \$8-10\:\text{mA}\$. You can tell by measuring the voltage across it. Target \$5-6\:\text{mA}\$, by changing its value.

Adjust \$R_4\$ (which can also be just a regular resistor, if you like swapping them in and out) so that there is about \$1-3\:\text{mV}\$ across \$R_7\$ and, separately, \$R_8\$. You can measure the voltage across them with a meter. If you have to short out \$R_4\$ to get the voltage drops that low, then get rid of \$R_4\$ entirely (shorting it) and start adjusting \$R_3\$ to make that work out. \$R_5\$ is not critical and you can completely remove it, if you want. (Still make the above adjustments, though.)

I set up \$R_{10}\$ and \$R_{11}\$ to provide a small amount of voltage gain. About a factor of 2.2, as you can see. Since the output cannot exceed \$\approx 1\:\text{V}_\text{RMS}\$, you should keep the input below \$450\:\text{mV}_\text{RMS}\$ with \$R_{10}\$ and \$R_{11}\$ set as shown. But you are free to adjust those ratios, too. Mostly, this is just about getting something that sounds okay when you run it.

I believe all of this should be within reasonable operating range of the LM358.

Now, you can un-ground your input and attempt to apply a small signal to it. Start small and work upwards.

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schematic

simulate this circuit – Schematic created using CircuitLab

Just go get an opamp and wire this up. The simplest design that give you enough loudness. Increase value of R2 and R4 to increase gain.

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