BJT Amplifier Help

Discussion in 'The Projects Forum' started by jrjones0109, Dec 6, 2012.

  1. jrjones0109

    Thread Starter New Member

    Dec 6, 2012
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    Hello,

    I am new to this forum. I am having trouble coming up with a woking design for my college level Electronic Devices and Circuits course that I am taking. Here is the problem:

    Design a 2-stage BJT Amplifier with a 3.5V peak-to-peak output. The input is a 70mV (RMS) signal. The load it will drive will be a 1 megaohm resistor. Minimize all noise.

    I recently built a single stage stable amplifier that amplified a 100mV signal at 1KHz to about 1.30V output, but I can't seem to come up with a dual stage amplifier. Would a diff amp work for this problem?

    Any help would be appreciated!
     
  2. tubeguy

    Well-Known Member

    Nov 3, 2012
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    What is the trouble you are having?

    When you build multiple stage transistor amps you usually need to couple one stage to the next with a coupling capacitor. And, the stage gains multiply.

    Why don't you post your single stage amp schematic and/or what you have tried so far.
     
  3. jrjones0109

    Thread Starter New Member

    Dec 6, 2012
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    I pulled this circuit off of the allaboutcircuits website. I am just having trouble calculating if this exact circuit will provide the right amount of gain that I am looking for to get the correct output voltage.
     
  4. tubeguy

    Well-Known Member

    Nov 3, 2012
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  5. absf

    Senior Member

    Dec 29, 2010
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    I simulated the circuit you posted.

    With an input of 1000Hz 70mV sine wave, you're not going to get 1.3V output at the speaker. All the DC voltages are listed there and the wave form is on the scope. Distortion is high and the bottom half of the sine wave is gone totally.

    I think you need a major rework on this circuit.

    Allen
     
  6. jrjones0109

    Thread Starter New Member

    Dec 6, 2012
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  7. jrjones0109

    Thread Starter New Member

    Dec 6, 2012
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    I found this one... it looks a lot better. What do you guys think?
     
  8. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Exactly right. These are links to help You learn to design.
    There are a couple of tips in post #2 :)

    Post #7 is the right idea.
    Tip: Peak to peak output is 2.828 x RMS output. Important for calculating gain. (3.5/2.828= ?? RMS)
     
    Last edited: Dec 10, 2012
  9. Audioguru

    New Member

    Dec 20, 2007
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    The first circuit you found wrongly has an 8 ohm load. When it has a 1M ohm load then its voltage gain is much too high.
    Do you know how to add negative feedback to reduce the gain?

    EDIT: The upside-down second circuit also has too much gain.
     
  10. jrjones0109

    Thread Starter New Member

    Dec 6, 2012
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    Do you reduce the magnitude/ and or ratio of RC and RE?
     
  11. jrjones0109

    Thread Starter New Member

    Dec 6, 2012
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    This is what I'm getting with my first test (using about 7V DC) this is just after the 1st stage. I tried to measure after the 2nd stage and it was completely messed up, I'll have to take a look at that. But as you can see the top is cut off of my output signal.

    Edit: for some reason the picture is upside down.
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    The capacitor parallel with RE increases the gain a lot. Try the circuit without the capacitor or add another resistor in series with the existing emitter circuit.
     
  13. Audioguru

    New Member

    Dec 20, 2007
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    1) Replace the antique transistors with 2N3904 ones.
    2) Bias the first transistor properly by changing the 220k resistor to 150k.
    3) Change the input level to 285mV peak or 202mV RMS.
    Then the output is 3.5V p-p.

    But the voltage gain is too low so you must reduce the value of the emitter resistor and change the biasing again.
     
  14. jrjones0109

    Thread Starter New Member

    Dec 6, 2012
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    Good news guys, I got it to work! Once took out the capacitors on the emitter out and replaced them with resistors it was pretty easy from there. I just had to tweak my resistor values for RC and RE and it ended up working really well! I was able to get all the way up to 4V (p-p) output before I got distortion.

    Now, there is a part 2 extra credit to this project. I will post my current schematic and details on part 2 shortly. It is a PA system.

    Thank you guys so much for your help, I couldn't have done it without you.

    Jason
     
  15. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Isn't it supposed to be in the "homework" forum?
     
  16. jrjones0109

    Thread Starter New Member

    Dec 6, 2012
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    This is the circuit that I got to work. Using a 20mV (rms) signal at 1kHz I was able to get up to about 4V (p-p) before I got distortion. Perfect.

    Now, I need to turn this into a PA system. I need to use my current class A amplifier to drive an 8 ohm speaker (the input would be a microphone). Basically, after talking with my professor I've determined that what I have now is essentially a "PreAmp." I need to build a power amplifier off of this to drive the speaker though... What i'd like to do is use an IC to accomplish this. Does anyone have any suggestions??

    Jason
     
  17. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Last edited: Dec 11, 2012
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