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I'm tring to build an audio amplifier. I decided to use a basic common emitter amplifier circuit as a starting point. Here it is:

http://i.stack.imgur.com/03j4o.png

I have some questions about the circuit.

1-) What transistor can I use in 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?

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I've editted the question a little bit. (2n2222 and speaker) –  user10002 May 28 '12 at 3:24
    
I'm waiting for answers to edited circuit. –  user10002 May 28 '12 at 8:32
    
Output impedance of the circuit is 8k (as the collector is near infinite), which is rather high for driving a 4ohm load / speaker. If you change the 8k resistor, you also have to fiddle with the other ones and when currents increase you might even need another transistor with higher current rating. This circuit works pretty well, but for load impedance of 12k, not for a 4ohm speaker. It might be useful as a buffer stage. –  jippie May 28 '12 at 11:17
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You apparently intend R1, R3, C1, and Q1 to be connected. If so PUT A JUNCTION DO THERE. Can't tell what is R1, R2, etc? ADD COMPONENT DESIGNATORS like a real schematic. –  Olin Lathrop May 28 '12 at 15:31
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3 Answers 3

There are a number of things wrong here

  1. There are no component designators in the schematic. This makes it hard to talk about, so I'll be more brief than otherwise.

  2. You should use junction dots at all points where nets connect. The four lines crossing left of the transistor is ambiguous.

  3. A quick calculation of the bias point not taking base current into account results in 1.3 mA thru the transistor, so your claim of 1.26 mA is plausible. However, this should tell you there is no way you're going to get anywhere near 1 W of output power. Even looking at the transistor as a perfect switch, the collector swing will be from 6.6 to 24 volts, for a swing of 17.4 volts. However all this is at a impedance of 8 kΩ. 17.4 Vpp sine wave is 6.15 V RMS. At the best case power transfer, the load is 8 kΩ so the voltage is half that, or 3.1 V RMS into 8 kΩ, which comes out to 1.2 mW. That's less power (although at a different voltaqe) than the MP3 player is likely putting out.

  4. The frequency response is a mess. The gain of the amp only 2.7 at low frequencies. This starts going up 3 dB/octave at about 50 Hz, but the high end is unpredictable because it relies on properties of the transistor you don't know. Then the cap in series with the speaker forms another high pass filter, but this also allows the speaker to load the output heavily probably having a reverse effect for part of the range.

  5. I can't even begin to guess what the 12 kΩ resistor accross the speaker is supposed to do. It makes no sense whatsoever.

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  1. 2n3904 or 2n2222 because it is generic model and aplication in your example is least demanding, no other reasons for the choice.
  2. It might be very valid voltage. The number is called by health safety and it might be much less than legacy 0.775v/1KOhm numbers.
  3. The speaker is any ordinary speaker, big or small. I will not argue with value of output resistor, again just because of health safety. Your estimate of output voltage is wrong, simply because beta belongs to device only, not the cascade with resistors and feedback cap. The output voltage will always be no greater than DC power supply. Practically, may be 10..20% of DC voltage. edit: ignore last paragraph since the schematics was changed after answer.
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I'm using ß = 150 to calculate Zin. –  user10002 May 28 '12 at 2:03
    
How can I choose transistor without DC analysis results? How do I know if it's in active mode or not? –  user10002 May 28 '12 at 2:06
    
You can simplify DC analysis by looking at edge cases: max power, max voltage, max current and pick an overpowered transistor part number. In your case worst power is certainly below 100mW, ~3mA, 24V. So nearly any part fits. No matter if mode is active or switched. –  user924 May 28 '12 at 2:16
    
Thanks.. Well, I'dont understand your answer to the second question. –  user10002 May 28 '12 at 2:19
    
2) 300mV is a good guess. I mean you are most likely correct. –  user924 May 28 '12 at 2:26
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I suggest you to add a transformer to transfer the power to the speaker, so you can have the primary on the collector and the speaker on the secondary, that at least guarantee you some more power transfer.

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