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I'm looking for some advice on driving an 8Ω 2W speaker near full power from a 3.3v DAC. I've put together a current amplifier using a rail-to-rail op amp and two BJTs, but it's not supplying as much current as I thought it might.

In an ideal scenario, I was hoping to be able to drive the speaker up to ~1.25W, but in reality I'm only getting about 0.3W when putting 3.3v into the positive input of the op amp.

3.3V / 8Ω = 412.5mA = 1.36W

Wondering if anyone has any suggestions for improving this circuit or any alternatives. The DAC is providing a 3.3v output and my available power supply is 3.3v so I would prefer any amplifier I use to have a unity gain.

Thank you!

current amplifier circuit

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  • \$\begingroup\$ Is that schematic correct? Your inverting input should be connected to the output for unity gain, not ground. \$\endgroup\$
    – brhans
    Apr 9 '20 at 16:13
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    \$\begingroup\$ Assuming your opamp is actually wired up correctly, your transistors are emitter-followers - you'll never get then to produce a rail-to-rail output. Best case is you'll get 0.6V to 2.7V (and that's probably optimistic). Then consider that that gives you 2.1Vpeak-to-peak, which equals 1.05Vpeak or about 0.74Vrms (assuming a sine wave). That results in less than 70mW to the speaker ... \$\endgroup\$
    – brhans
    Apr 9 '20 at 16:14
  • \$\begingroup\$ Divide that 3.3 V by 2 due to AC coupling though that capacitor and divide that with sqrt(2) to get RMS. Still loud enough? \$\endgroup\$
    – winny
    Apr 9 '20 at 16:16
  • \$\begingroup\$ @brhans you are correct I diagramed the schematic incorrectly. The circuit is wired up properly. \$\endgroup\$
    – AngryPeaches
    Apr 9 '20 at 16:32
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You will get the best results if you use a Class D full-H driver.

More here: https://www.analog.com/en/analog-dialogue/articles/class-d-audio-amplifiers.html

And here: https://os.mbed.com/users/4180_1/notebook/tpa2005d1-class-d-audio-amp/

Also, consider using a 4 ohm speaker instead.

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  • \$\begingroup\$ Thank you. Looking at the datasheet of the TPA2005D1 it looks like it has a minimum gain of 2. Do you know of any amplifier ICs that have a unity gain? Of course I could always half the signal from the DAC, but it seems like this might cause some distortion compared to not applying any gain. \$\endgroup\$
    – AngryPeaches
    Apr 9 '20 at 16:34
  • \$\begingroup\$ It is a unity gain, but differential. With a single-ended input the gain will be 1V/V on the one swinging input. \$\endgroup\$
    – hacktastical
    Apr 9 '20 at 16:47
  • \$\begingroup\$ Thank you. That's a big help. Will start looking into Class D amps. \$\endgroup\$
    – AngryPeaches
    Apr 9 '20 at 16:58

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