Electronic – Two-stage amplifier

amplifierdarlingtonemitter-followerRFtransistors

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?

Best Answer

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.