Common Emitter Audio Amplifier Question

[kzriwoga]

I'm tring to build an audio amplifier. I decided to use a basic common emitter amplifier circuit as a starting point and added a speaker to it.

I have some questions about the circuit.

1-) Is 2n2222 ok for this circuit?

Results of DC analysis: Vce = 10.06V and Ic = 1.26mA (I guess, these values will be used for choosing the transistor.)

2-) What's the output voltage of a standart mp3 player? Can it be 300mVp as I read somewhere?

3-) When I assume ß = 150 and input = 300mVp , the output is -2,13Vp . I guess it gives 1w power to speaker. Is there something wrong?

4-) Is connecting speaker parellel to output R ok?

Thank you..

 

[dselec]

Check out this site
https://www.electronics-tutorials.ws/amplifier/amp_5.html

 

[kzriwoga]

Check out this site
https://www.electronics-tutorials.ws/amplifier/amp_5.html

That circuit has different mechanics. It puts a DC current thru the speaker. And there is no answer to my questions.

 

[godfreyl]

2-) What's the output voltage of a standart mp3 player? Can it be 300mVp as I read somewhere?

Yes, 300mV peak is reasonable. MP3 players can give higher output than that with loud recordings, but there is a volume control to turn it down.

3-) When I assume ß = 150 and input = 300mVp , the output is -2,13Vp . I guess it gives 1w power to speaker. Is there something wrong?

Yes.
Signal current into base = 300mV pk / 90K = 3.33uA pk
Signal current out of collector = 3.33uApk * 150 = 0.5mA pk

With no load connected:
Output voltage = 0.5mA pk * 8K = 4V pk

With 12K load added:
Output voltage = 0.5mA pk * 8k||12K = 2.4V pk

With the 4 Ohm speaker:
Output voltage = 0.5mA pk * 4 Ohms = 2mV pk

1-) Is 2n2222 ok for this circuit?

Not if you want 1 watt peak power to the speaker. 1 watt peak into 4 Ohms = 2V peak * 0.5A peak, so the amplifier needs to give +-0.5A current to the speaker.

To do that, the DC current through the transistor has to be more than 0.5A because it also has to give signal current to the collector resistor.

If a 10 Ohm collector resistor is used, that will need +-0.2A of signal current, so total signal current from the transistor = +-0.7A, so a DC current of 0.75A through the transistor will be just enough.

If the DC voltage across the transistor is 4V, then the power dissipated by the transistor is 0.75A * 4V = 3 Watts.

You need a transistor that can dissipate 3W of power and conduct 0.75 amps average current, and 1.5A peak current. That is too much for a 2N2222. A medium power transistor such as a BD139 should be OK if it is on a heatsink.

The next problem is that the current gain of the BD139 will be very low (maybe 20), so you need another transistor between the MP3 player and the BD139.

 

[FvM]

I think an obvious conclusion from the facts presented by godfreyl is that it doesn't make much sense to use a class A amplifier with a speaker.

 

[godfreyl]

Well, it's a choice. Single-ended class A is the least efficient kind of power amp, but there are actually some commercial examples, e.g. some of Nelson Pass' "First Watt" designs. Push-pull class A is more efficient and much more popular, though.

---------- Post added at 15:09 ---------- Previous post was at 14:45 ----------

Just for fun, here's an article describing a "simplest possible" one-transistor power amp, and a couple of variations: De-Lite Amplifier.

It uses a MOSFET as the amplifying device (to get a high input impedance), and a light bulb for the drain resistor (because light bulbs are cheaper than power resistors, and look prettier).
:-D