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My goal was to amplify audio signal coming from Raspberry PI's audio jack, so it is louder. I have basically no clue what I am doing, but it seems to work - when I connect the speaker directly to the audio jack, it is very weak, but connecting with transistor makes much louder sound.

Is my circuit correct?

Or am I going to destroy the speaker or even raspi?

How comes there is no GROUND needed in my circuit?

My transistor is CTBC 640 My speaker is 8 ohm, 0.25W

enter image description here

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    \$\begingroup\$ but it seems to work I doubt that. Many have tried driving a speaker using only one transistor and they have also found that at best that works very poorly. To amplify an audio signal for a speaker use an Audio Power Amplifier Module. Your circuit does have a ground, the shielding of the audio jack plug is grounded. Indeed this "circuit" could damage the transistor and maybe the speaker. The RPi should be relatively safe. But anyway: use a ready-made module instead of this horrific "circuit". \$\endgroup\$
    – Bimpelrekkie
    Nov 29, 2018 at 12:03
  • \$\begingroup\$ Could you suggest a link to such module? \$\endgroup\$
    – Tomas M
    Nov 29, 2018 at 12:08
  • \$\begingroup\$ Sure, here's an example: ebay.com/itm/… Find more by searching for "LM386", that's an Audio Amplifier Chip which is used on these modules. Buy a module not only the chip as even that chip needs some extra components to make it work. \$\endgroup\$
    – Bimpelrekkie
    Nov 29, 2018 at 12:10
  • \$\begingroup\$ Note that the LM386 is just one of thousands possible chips that fulfill the role of an audio amplifier. It's just really old, and hence cheap, and a lot of chinese factories have ready designs to flood ebay with, and so, the complete boards containing that are cheap as dirt. Better amplifiers abound. Since Audio amplification for low-powered speakers is about the easiest analog problem you can have, basically every semiconductor produces offers these (they run these kinds of chips when their fabs aren't busy producing higher-margin wafers to not have production capacity==capital idling away) \$\endgroup\$
    – Marcus Müller
    Nov 29, 2018 at 12:15
  • \$\begingroup\$ One example is TI: ti.com/audio-ic/amplifiers/speaker-amplifiers/products.html \$\endgroup\$
    – Marcus Müller
    Nov 29, 2018 at 12:15

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You have (brilliantly) managed to implement an "emitter follower.

This effectively amplifies current that can be provided to a load - with a decrease in voltage of about 0.6V across the transistors base-emitter junction.

The "jack" outputs signal and ground. A PNP transistor (such as you have shown) required a collector supply that is negative relative to the emitter - which is what you have got.

The circuit is (no surprise) not ideal but you are unlikely to damage the RasPi.
However, this is not certain - and using a formal amplifier IC is liable to be safer and work (even) better.

Wikipedia Common collector - which this circuit is.

Garglabet images - PNP emitter follower - each image is linked to a (probably) relevant web page.

enter image description here

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  • \$\begingroup\$ GPIO pin 1 is the 3.3V rail of the Pi. It isn't one of the digital outputs. \$\endgroup\$
    – JRE
    Nov 29, 2018 at 12:50
  • \$\begingroup\$ @JRE Thanks - I assumed it meant 1 of eg [0...7] in an I/O bank. Will amend answer. \$\endgroup\$
    – Russell McMahon ♦
    Dec 2, 2018 at 6:28
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That is not good.

  1. You have no current limiting resistor to the base of the transistor. This can cause the transistor to fail (burn out.)

  2. You have DC going through the speaker at all times. This can damage the speaker.

  3. You have nothing to limit the current through the speaker and transistor. If the transistor fails short, you have all the current the 3.3V output of the Pi can provide going through the speaker. This could cause your Pi to crash. If the speaker lasts long enough before burning out, it could damage the Pi.

As for the ground, that's through the second wire of the earphone jack.

There are simple, 1 transistor amplifier designs on the internet. They work well enough.

There are many more circuits using dedicated amplifier chips that are as easy to build as a single transistor amplifier, but which will work much better.

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