Common Emitter Amp drive speaker

Discussion in 'General Electronics Chat' started by mbird, Feb 17, 2009.

  1. mbird

    Thread Starter Active Member

    Jan 29, 2009
    Hi --

    I made a common emitter amp (just for hobby/exploring in order to learn how to make these amps, explore transistors, how the biasing works, how to calculate gain, etc.) That worked well and I simulated it in ORCAD trial version 16.

    Now I'd like to try this out on a breadboard. I assume I can try with a small microphone as the input but I realize I cannot just add a small 8Ohm speaker to it (when I do calculations with the added 8Ohm speaker it looks like my gain would drop from about 200 to about 2!). What is the best way to connect this to a small speaker?

    Two ways I found are...
    -Emitter Follower
    -Op Amp (if so which one -- the LM386?)

    I would like to try with emitter follower to learn about that so I tried a simple emitter follower (unity gain with no biasing) but I am suspicious that my setup is not quite correct. I attached what I have so far.

    Thanks for any ideas or help on this. I have found a lot of info on the internet to get me at least to understand the Common Emitter amplifier setup so that is fun!

    Thank you!
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    Last edited: Feb 17, 2009
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    For a Class A amp, about your only option is with a transformer.
  3. Audioguru


    Dec 20, 2007
    An opamp has a max output current of about 30mA. Then its max undistorted power to an 8 ohm speaker is only 0.004mW which is nothing.
    An LM386 little power amp IC has a max output current of 0.32A so its undistorted output to an 8 ohm speaker is 0.4W, like a cheap clock radio.
  4. flat5

    Active Member

    Nov 13, 2008
    If any of your caps should be 1000uf it is C3.
    Then you just might hear something from the speaker.
    Of course if your speaking voice is ludicrously high, .1uf
    might be sufficient.
    Last edited: Feb 17, 2009
  5. mbird

    Thread Starter Active Member

    Jan 29, 2009
    Hi --

    Am I correct that from what you said I can put a small microphone or output from my computer line-out or output from some other device like an iPod into my simple Common Emitter amp and then take the output of that amp and run it to an LM386 and that will give enough for my small speaker (8 Ohm 250mW) to hear some sound.

    I realize this would not be ideal sounding but my goal is to just have some fun in a hobby mode and study how biasing, gain, etc worked for a common emitter amp so I just wanted to take it from the simulation to actually hear something by piecing it together on a breadboard.

    If this is the case then that will be a bonus since will be a chance to play/learn about the LM386.

    Thank you for your time and ideas!
  6. mik3

    Senior Member

    Feb 4, 2008
    Make RL2 10K and C3 470uF.
  7. italo

    New Member

    Nov 20, 2005
    You may get better results if you eliminate the last stage 200 Ω and the cap and tie the speaker to ground. add a limiting resistor in series to the speaker. all those extra components rub you of power to the speaker. and the previous gain is only 2.5 not much but then again it is an experiment.
  8. flat5

    Active Member

    Nov 13, 2008
    If you can get an audio output transformer that would be the best way to interface with the speaker in this simple circuit.
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
    Make RL2 10k and the emitter follower becomes a half wave rectifier. RL2 has to sink speaker current during negative transitions of the voltage on the base of the emitter follower. This is the reason for push-pull output stages.
  10. mbird

    Thread Starter Active Member

    Jan 29, 2009
    What I have discovered from all the above. Please correct/comment if my observations are correct/incorrect...

    I made a common emitter amp (just hobby/fun/exploring) that I then want to connect to a microphone/speaker.

    My assumptions and what I gleaned from the above posts...


    I cannot connect it directly because that would cause my gain to drop from about 200 to about 2 (I did not try it but I realized that from doing the gain calculations).

    If I wanted to just drive the speaker in one direction then I could use an emitter follower. I could have the speaker connected to the emmiter follower like this:
    Base--RL--Speaker--ground (and use RL to control the current)

    I'd really like to drive the speaker in both directions so I have some options:
    1.) use an audio output transformer
    2.) instead of emitter follower use two transistors in a push/pull way
    3.) use a variety of more complicated next stage
    4.) do not use an Op Amp since it is too low power to drive a speaker (could use LM386 probably just for this test)

    Please let me know if these conclusions are correct and if there are any other possibilities.

    I am just a hobbiest -- decided to play/learn transistors then got up to Common Emitter amp and would like to just make a simple breadboard circuit to use my common emmiter amp to amp a microphone to a small 8 ohm 250mw speaker.

    Thank you for your ideas and time!!!
  11. flat5

    Active Member

    Nov 13, 2008
    You need more than one stage of amplification.
    Probably three transistors will be needed.
    You would have better success using a headphone.
    Many are 32 ohms and this alone will help.
    Good ones are much more sensitive than a speaker.
    I'd say two stages of gain would be enough to play with.

    Your other notes seem correct to me, except 3-2 in that you still need more gain but it is a better way to drive a speaker.