How can I amplify an a/c signal from a function generator using a BJT?

Discussion in 'Homework Help' started by swears, Oct 29, 2009.

  1. swears

    Thread Starter New Member

    Oct 29, 2009
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    0
    I've built an amplifier using a 2N3906 PNP transistor, however, my output is smaller than my input.

    I am inputting a 1volt a/c sin wave @ 1kHz and my output barely shows up on the oscilloscope.

    Here is a picture of the circuit I am trying to build: Fixed Bias.jpg

    Vcc is 5V
    C1, C2 are .001μF
    Rb, Rc are 1kΩ

    Am I doing something wrong? I've been looking at this for a week now, changing capacitors and resistor values and I still can't figure out what I am doing wrong.

    Please note, I don't need to use this circuit specifically ( I picked it from the book because it said it was simple), I just need to use a transistor to amplify a sin wave.

    Any help is greatly appreciated.

    Thank you.
     
  2. ELECTRONERD

    Senior Member

    May 26, 2009
    1,146
    16
    Your circuit evidently isn't amplifying but rather attenuating.

    Your first problem is using a PNP transistor, the schematic ask's for a NPN transistor; they asked that for a reason.

    Your second problem is having both the collector and base currents the same. The base resistor should be much higher than the collector current. In fact the base current should be 1/10th the collector current.

    Austin
     
  3. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    The circuit you show uses a NPN transistor.
    A PNP transistor will not work.
    The polarity is wrong.

    The base resistor is also to small, the transistor would be in saturation.
    The attached schematic will probably have more success.

    [​IMG]

    Greetings,
    Bertus
     
  4. swears

    Thread Starter New Member

    Oct 29, 2009
    5
    0
    Thanks.

    I can't wait to go try this next time I go to the lab.

    I'll let you know how it goes!
     
  5. swears

    Thread Starter New Member

    Oct 29, 2009
    5
    0
    Bertus, I gave your circuit a shot today in the lab, however, it still doesn't seem to work. I've attached pictures of the actual hardware circuit with labels. Perhaps, there is a small mistake that I'm making.

    I wasn't able to get 1μf capacitor, the closest I could get was .001μf. I don't know how big of a big difference this would make.

    Let me know what you guys think.



    Thanks.
     
  6. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
    Two problem you have is the 1k resistor should be connected to the NEGATIVE 5V as well as the 100k, since you are using a PNP transistor. You had the 1k effectively connected to the Base of the transistor and the Collector of the transistor.

    I noticed you labeled the Vcc as Positive 5V, it needs to be negative 5 V

    As far as using the 0.001 uF vice the 0.1 uF, you increased the capactive reactance by a factor of 100, which reduced your available signal by a factor of 100.
     
  7. swears

    Thread Starter New Member

    Oct 29, 2009
    5
    0
    OK, I fixed the circuit (somewhat). I now have Vcc as -5 Volts and I fixed the positions of the resistors.

    It seems that the transistor is acting like a switch rather than a amplifier. It turns on when the Vin is totally negative. Here is what my output looks like:
     
  8. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    1,159
    Yes it is. What is the amplitude of the AC signal entering this circuit? It has to be a low level, 20 mV pk-pk. Lower the amplitude till you get both halves of the output sine wave.

    Watch the oscilloscope as you adjust the input amplitude.
     
  9. swears

    Thread Starter New Member

    Oct 29, 2009
    5
    0
    Joe,

    I lowered the amplitude from 1 volt to .1 volts, and now it amplifies!

    The only problem now is that the output doesn't look anything like the input. I believe the transistor is supposed to invert the sine wave and amplify it, but the output looks more like a square wave. Is it saturated?

    Also, this may be a dumb question, but how can I calculate the gain without actually building this. Is their an equation? I think it has something to do with the ratio of the resistors.

    New output is attached:
     
  10. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
    You are still overdriving the transistor. Lower it until you see the undistorted amplification. It will be in the 50 millivolt peak-to-peak range.

    http://www.allaboutcircuits.com/vol_3/chpt_4/1.html has information on transistors and transistor amplifiers.
     
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