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Common-collector buffer

  1. Jul 29, 2011 #1
    hi
    i am having my frist attempt on designing a common- collector transistor buffer which has a unity gain of 1. i am using lt spice to check the input and output of my buffer, however i am not sure how to determine if my buffer is working. if my input is a square wave with 3 volt amplitude then out should the output of my buffer look like?
     
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  3. Jul 29, 2011 #2

    vk6kro

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    A square wave is not a good test signal for testing an analog amplfier.

    Try using a sine wave. If the output is exactly like the input sinewave, although slightly smaller in amplitude, then the emitter folower is working OK.
    The gain of an emitter follower is slightly less than 1.

    You can also measure the input current and then work out the input impedance of the amplifier. It should be very high.
     
  4. Jul 29, 2011 #3
    i have designed the buffer to have a input impedance of 57.4k, output impedance of 50.9 ohms and gain of 0.99 are those specs good for a buffer and by the way this is the output i get from ltspice
     

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  5. Jul 29, 2011 #4

    vk6kro

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    You would need to insert a capacitor between the signal generator and the base of the transistor. Or, you can put a DC offset into the parameters of the signal generator.

    The voltage on the emitter of the transistor should be about half the supply voltage.

    LT Spice has a nice feature where you can vary a DC voltage from the signal generator (without the series capacitor) and observe the output. You can use this to set up the DC conditions of the amplifier so that when you drive it with a sinewave, the sinewave fits on the straight line slope between the positive supply voltage and ground.
     
  6. Jul 29, 2011 #5
    i have added a capacitor before the transistors base but still nothing is my configuration correct? added schematic
     
  7. Jul 29, 2011 #6

    vk6kro

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    There doesn't seem to be a schematic attached. Could you try again?

    Did you understand about the DC sweep trick?
     
  8. Jul 29, 2011 #7
    sorry i taught i attached it.
     

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  9. Jul 30, 2011 #8

    vk6kro

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    Do you have the base and emitter voltages?

    You get these by moving the cursor until it becomes a test probe. Then click.

    You could try decreasing the size of the top base resistor. Try 50 K.
     
  10. Jul 30, 2011 #9
    hi again
    i tried decreasing the size of the top base resistor but it didnt work so i tried changing the capacitor values and now my output looks like this(output is the green colour). so is the buffer working because in your above statement you said

    If the output is exactly like the input sinewave, although slightly smaller in amplitude, then the emitter folower is working OK.

    since i havent used a sine wave here but a square wave it seems the output is smaller in amplitude then the original input. is this correct?
     

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  11. Jul 30, 2011 #10

    vk6kro

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    You need to use a sine wave, because a square wave output could be the input square wave or it could be the input square wave with the top or bottom clipped off. You couldn't tell.

    A sinewave output would indicate that the amplifier is working OK, because any clipping would be obvious.

    You seem to be using very large input signals. You should set the input to 0.2 volts so that you will get an input signal of 0.4 volts peak to peak.

    However, MEASURE the DC voltages at the base and emitter and post them here.

    Incidentally, in the circuit diagram, it shows that the base resistors are not connected to the base. You need to fix this so that there is a blue square where they join like the one at the top of the emitter resistor.
     
  12. Jul 30, 2011 #11
    It appears that R2 and R3 are not connected to the base.
     
  13. Jul 30, 2011 #12
    omg i cant believe i did not see that. i have now attached the resistors to the base and this the output i get (red/green graph where input is red and output green):

    i wasnt sure to measure the dc voltage using ltspice so i plotted the base voltage against the emitter voltage ( blue/green graph where blue is the base plot and green the emitter plot)
     

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  14. Jul 31, 2011 #13

    vk6kro

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    It looks like you are overdriving the amplifier.

    Try reducing the signal generator output to a quarter of what it is now.

    The output should be a sinewave like the input.

    You can measure the DC voltages by reducing the input signal to ZERO and observing the voltages on the graph.

    You could also try again to reduce the value of the top base resistor. Try 100 K then 50 K.
     
  15. Jul 31, 2011 #14
    DC voltages of base = 3.1V
    DC voltage of emitter =2.5V
     
  16. Aug 1, 2011 #15

    vk6kro

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    That sounds ideal.

    If you click on the voltage generator and get to the "sinewave" description, set the frequency to 1000 Hz, the amplitude to 0.1 volt and the number of cycles to 5000, you can then run the simulation again and see if you get a sinewave out.
     
  17. Aug 1, 2011 #16
    new input and output does this seem correct?
     

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  18. Aug 1, 2011 #17

    vk6kro

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    Yes, that looks good.
     
  19. Aug 1, 2011 #18
    thank you

    zak 8000 wheres ur manners
     
  20. Aug 1, 2011 #19
    thanks for your help
     
  21. Aug 2, 2011 #20

    vk6kro

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    The interesting and useful thing about an emitter follower is the change in mpedance that it allows.

    If you changed the emitter capacitor to 100 uF (so that its reactance was negligible at 1000 Hz), you could then test various resistors as the load and find out how low you can go before the output drops to half of the input, or gets distorted.
     
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