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Schematic

This is my first time designing a circuit by myself :)

The input of this circuit is the output jack of my MP3 Player.

The output of this circuit is 4-Ohm loud speaker (sometimes I connect 8-Ohm speaker).

The first stage is the common emitter mode and the second one is common collector mode. I wanted to make a power amplifier.

  1. How to get rid of thermal runaway of a transistor? Actually, The second transistor of the Darlington pair Q2 gets very hot in few seconds.

  2. Is this circuit able to amplify RF signals (especially FM frequency)? If so, where can I connect the cap, coil (tank circuit) and the diode?

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    \$\begingroup\$ It's not a perfect design, but it is very good for learning and understanding how things work. I think you can fix the transistor heating up largely by adding a series capacitor with the speaker. Say 470µF (or higher) to start with. It's RF properties will probably be disappointing. Apart from that, use your multimeter, measure voltages in the circuit and try to improve the circuit by experimenting with different resistors and transistors. A great way to learn! \$\endgroup\$
    – jippie
    Sep 12, 2014 at 15:57
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    \$\begingroup\$ Perhaps a series cap on the input too. 0.1 - 1 uF. \$\endgroup\$
    – George Herold
    Sep 12, 2014 at 16:28

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If you disconnected the speaker you'd find that the output transistor probably doesn't get warm - the speaker is like a resistor to ground of just a few ohms and this will be taking maybe 500mA thru Q2 without any input signal or sound being present. Like @jippie says in his comment, try adding an electrolytic capacitor in series with the speaker to prevent DC current flowing thru the speaker - speakers are not meant to have dc thru their coil anyway - it offsets the diaphram and can add distortion.

Amplifying radio frequencies is possible with this circuit but the right type of transistors will be necessary if you are aiming for 100MHz operation. I'll also add that you can't effectively demodulate F.M. with a simple diode rectifier circuit - it's more complex for frequency modulation but OK for amplitude modulation.

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