A CE amplifier I built successfully

Discussion in 'General Electronics Chat' started by alphacat, Nov 9, 2009.

  1. alphacat

    Thread Starter Active Member

    Jun 6, 2009
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    I designed and built the following CE amplifier.

    DC operating point:
    Measured VCE = 2.382Vdc.
    Measured VBE = 0.6Vdc.

    Small signal data:
    Measured input: vin = 10.2mVpeak
    Measured output: vout = 0.955Vpeak.
    => voltage amplification of -93.6 (there was a 180 phase between input and output, as expected).

    I have several questions pleaes:

    1. Could you please tell me your opinion about my design? how would you improve it?
    I'm a bit worried that i had to use such a large RBASE (it took the capacitor long time to charge).
    I chose such large a resistor since for that value, Vout was around 2.5V, which was my goal in order to receive a large output voltage swing.

    2. Is there any way to derive β from my measurements?

    3. Is it possible to connect a speaker to the vout node (from the Collector to Ground) and hear the sound which the signal generator (input) creates?
    Would it work?


    Thank you very much!

    [​IMG]
     
    Last edited: Nov 9, 2009
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Missing an emitter resistor, which stabilizes gain even for different transistor temperatures.

    The output impedance is too high to drive a speaker efficiently, not enough current could go through the 10k Rc to drive an 8Ω speaker.

    In addition to an emitter resistor, a capacitor should be placed in parallel with the emitter resistor so the signal doesn't change the calculated gain (Rc/Re)

    The input with decoupling capacitor should be to the left of the base resistor, with one end of the decoupling cap at the junction of R1 and R2 biasing resistors, the other end of the cap connected only to the signal generator.

    That's off the top of my head.

    Here is an auto-designed CE audio amp from multisim:

    [​IMG]
     
    Last edited: Nov 9, 2009
  3. alphacat

    Thread Starter Active Member

    Jun 6, 2009
    186
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    Thank you very much.
    How can you know what is the base current in the configuration you offered?

    Moreover, you said that there isnt enough current to drive the speaker.
    You mean that not enough AC current would pass through the speaker if i connect it in prallel with the CE junction?
     
  4. hobbyist

    Distinguished Member

    Aug 10, 2008
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    The amp schem. "thatoneguy" shows is the classic all around cleass A amp,

    But for this being your first attempt at a class A amp design,
    According to your stats, looks like it was a successful design,

    you got the bias voltage at the output, close to half supply, and you got a voltage gain, at the output, so even though it is a rough design, you still got it to work.

    Now study what "thatoneguy" said in his post and youll pick up real quick and you should have a good reliable amp designed sometime soon.
     
  5. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Base current is around 200uA, gain of only 3. out of boredom, I ran the CE Wizard to build one with a gain of 3 while reading this thread.
     
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Ib=[ (RB1*Vcc)-Ube*(RB1+RB2) ] / [ Re*(β+1)*(RB1+RB2)+(RB1*RB2) ]
    So for β=150; Vbe=0.68 we get
    Ib≈14.76uA
    Ic
    2.2mA
    Voltage gain Au=Rc||RL/re≈40*Ic*(Rc||RL)≈2.8V/V

    And both circuit are horrible and nobody besides amateurs use this simple circuit.
     
  7. electr

    Active Member

    May 23, 2009
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    @Jony
    You used beta in order to determine IB(dc), but you can never know the exact beta since beta varies for different operating points.


    About the speaker, you said that there isnt enough current to drive the speaker.
    You mean that not enough AC current would pass through the speaker if i connect it in prallel with the CE junction? (using a coupling capacitor to block DC current).

    If so, how would adding an Emitter resistor RE help?
    That would only decrease the current gain of the amplifier.
     
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    If you have it built, hook a speaker up to it with a signal generator input. It won't be terribly loud, but it will be audible. The Re is for long term stability.

    To get power that will produce good volume, one ends up working with push-pull amplifiers, but for a 1st shot at a Class A, you did very well!
     
  9. electr

    Active Member

    May 23, 2009
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    Thank you very much, i'll try it out.

    Sorry for writing from a different user name, its the user name there's here at work.
     
  10. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    If you proper design the the circuit Ib<<Idz (current that is flow through R1 and R2) then Ic is almost independent form β
    For example RB1=4.7K; RB2=3.3K; Re=1K; Vcc=5V; Vbe=0.67 and for β=100 we get
    Ib_= 22uA and Ic=2.2mA
    for β=400
    Ib= 5.6uA ---> Ic=2.225mA
    As you can see Ic is almost independent of β despite that Ib changes in a wide range.

    RE don't change the current gain.
    RE reduces the voltage gain of a amplifier.
    Of course, this reduction of a voltage gain occurs only if we don't use Ce capacitor.
    Amplifier with RE but without Ce has a smaller voltage gain but has more input impedance and less harmonic distortion. Because RE apply a negative feedback to the circuit and that helps a lot to reduce the distortion.
     
  11. alphacat

    Thread Starter Active Member

    Jun 6, 2009
    186
    0
    Thank you very much for this explanation.

    The AC current gain is equal to:
    hfe / (1 + hfe*RE)
    So surely the current gain is degraded when using an Emitter resistor.

    Agree.
     
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