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Transistor Amplifiers

  1. Mar 24, 2008 #1
    I want to amplify the current/voltage from a music source. Specifically, I want to use the varying signal to vary the light intensity of a system of LED's connected in parallel.

    Basically, I want to create pulsating LED's. What I'd thought of was using a sound signal from my ipod to do so. I dont think that the amplitude of current/voltage of the signal obtained from the ipod directly will be enough to observe an appreciable variation in intensity, so first I want to amplify the signal, and then use its current/voltage variations to cause the fluctuation in intensity of the LED's.

    I will be connecting about 5-10 LED's in parallel (with a 1K-ohm resistor in series with each LED), connect this system across a 12v DC supply and connect the amplified audio signal in series with the battery. I believe this would work, but I dont know how exactly to amplify the signal or how much to amplify it by. I know how the transistor amplifier works, but the specifics (collector voltage, ratings of transistor, if any) elude me.

    As always, all help is appreciated.
  2. jcsd
  3. Mar 24, 2008 #2


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    Staff: Mentor

    Sounds like fun. You would generally use the common-emitter configuration with an NPN bipolar transistor acting as the "pull-down" active element, pulling current down through the LEDs and resistors to ground. In this CE amp tutorial:


    you would put your LEDs + Resistors in the position of R3, and not bother with the C2 output capacitor. R1 and R2 bias up the transistor, and C1 is used to couple the signal (your audio) into the transistor.

    This is a fairly good simple intro to CE amps as well:


    You might not get enough gain out of just one stage of CE amp -- I don't know what the voltage levels are out of your iPod or whatever. If it didn't have enough gain, I'd add an opamp as the first stage, and the follow that with the LED drive CE amp stage.

    Have fun! If you end up with specific questions in your circuit, post what you've got so far and we'll offer suggestions. It will definitely help if you can tell us what your signal levels are out of your audio source -- do you have an oscilloscope, or access to one?
  4. Mar 28, 2008 #3
    Ive found the value of the input voltage to be between 20mv to 120mv depending on the song being played, and this is the circuit I'm using:
    http://www.geocities.com/tomzi.geo/2tr_amp/2tr_amp.htm .

    The difference is that the transistor Im using is different and so are the resistances. The power supply is also a 12v battery and not a 5v one. How do I decide the values of the resistances? The transistor Im using is b547 NPN.
  5. Mar 28, 2008 #4


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    Staff: Mentor

    Yuck.... That's not a very good use of two transistors to make an amplifier, IMO.

    A much better way to go would be to use an opamp as the first stage to give you good gain, and follow that with an NPN transistor to drive the LEDs. Have you considered doing it that way instead?
  6. Mar 28, 2008 #5
    I know a lot of ipods use wolfson decoder/amplifier ICs

    WM9713 has a datasheet available

    The headphone output of this chip is designed to drive a 16 or 32 ohm headphone load. This may be something to take into consideration depending on how you're going to set up your amplifier.
  7. Mar 29, 2008 #6
    Err... opamps? I dont really know how to use them. How would I go about making one? Ive looked at the circuits available on wiki and they look pretty complicated. Should I just buy one?

    If the chip is used to drive a 16-32 ohm headphone load, then should I add a resistance in parallel to my LED circuit to make the load equivalent to 16-32 ohms?
    Last edited: Mar 29, 2008
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