How to calculate transistor perimeter

Discussion in 'General Electronics Chat' started by Thevenin's Planet, May 25, 2014.

  1. Thevenin's Planet

    Thread Starter Active Member

    Nov 14, 2008
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    Hello
    I am attempting to build a phase shift oscillator.To get the capacitors and resistors to turn the out put around to in-phase. The internal collector resistance is needed.Also the internal Emitter resistance is needed.Not very good at finding the matrix or determinants.The hybrid method uses a generator to obtain the hoe,hr,hie,etc.Is there a way to get these capacitors and resistor to produce a three(3) hertz oscillation besides the methods stated?
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Schematics please?
     
  3. Thevenin's Planet

    Thread Starter Active Member

    Nov 14, 2008
    183
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    This is the one.

    [​IMG]
     
    Last edited by a moderator: May 27, 2014
  4. Thevenin's Planet

    Thread Starter Active Member

    Nov 14, 2008
    183
    1
    Maybe I should ask this question.Should the Vce be half the Vcc?If so what should determine the output current,that is high or low or some amplifier particular class. Or should the output voltage across the vce be more important to determine the feed back across the high pass filters?
     
  5. Thevenin's Planet

    Thread Starter Active Member

    Nov 14, 2008
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    Why is there no response to this thread ?
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    I can think of several reasons;

    It really isn't a math problem, but an electronics problem.

    If the emitter swamping capacitor C4 wasn't there the input impedance would be easy, but as it is you almost have to have a data sheet. Given how much variation there is in a transistor family it would be only an approximation or worse.

    The output impedance of this circuit is not important, because the output will be 180° out of phase with the input no matter what the impedance is.

    So the parameters you need are the input impedance and gain. Gain can be Rc/Re (R6/R4), but C4 means the gain will be whatever the transistors gain is at that frequency. Again, data sheet required with all the uncertainties that adds.

    Op Amps are much simpler to predict overall.

    If you want I can move this to the Electronics Chat Forum?
     
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  7. Thevenin's Planet

    Thread Starter Active Member

    Nov 14, 2008
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    I did assume that it was possible that I can get some feedback on, How to find the appropriate capacitors that would give me this Gain or Impedance across the the high pass three filter stages to achieve the three hertz oscillation. Since it seems to use matrix math,I regarded the math forum as the appropriate site to get a better understanding.But if its better to place this topic in the Electronic Chat Forum for a variation of feedback I agree.And thanks again.
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    You can solve this with or without matrix math. Without getting into matrix algebra, you can calculate the R and C values for a 60 degree phase shift at 3 Hz, and with those values calculate the attenuation through one C-R network. That attenuation times 3 is almost amount of gain the transistor has to make up to sustain oscillation. The transistor has to have extra gain to make up for network loading of the collector and base loading of the network.

    I recommend deleting C4 and using R6 and R4 to make the transistor do what you want it to do.

    There are many websites that explain the phase shift oscillator, and some have calculators. It is a beginner's favorite because the reason it oscillates is easier to understand than a Hartley oscillator

    ak
     
    Last edited: Jun 20, 2014
  9. BobTPH

    Active Member

    Jun 5, 2013
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    Another variation is to use a potentiometer as the collector resistor with the tap being used for feedback. You can then adjust the gain to get a good clean sine wave.

    Bob
     
  10. Thevenin's Planet

    Thread Starter Active Member

    Nov 14, 2008
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    How do I do that ,THAT IS, CALCULATE RESISTANCE AND CAPACITANCE? I FOUND this FORMULA ;H(s)=Vo/Vi =j2*Pi*F*R*C/j*2*Pi*RC+1. Also,
    H(jw)=sqrt[(wRC)^2]/sqrt(1^2+(wRC)^2))=
    (wRC)/sqrt (1+(w^2)*(R^2)*(C^2).
    The Transfer function.In my booK the amplification or Beta.I notice that Time constant is involved.Also Radian * time constant =frequency.Is this the Magnitude and degree leading of the high pass filter?hat is the current leads the volt across the capacitor?
     
  11. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hello there,

    C4 could reduce input impedance quite a bit, and since we only need a gain of 29 you probably dont need that. If you really do then that's another story.

    With the Vcc bias resistor high relative to the other resistors, the R*C value is exactly 1/(6*sqrt(6)*pi), and the gain of the transistor part has to be set for a gain of 29 or a little above that.

    The basic idea is to get the phase shift of the three RC pairs to equal 180 degrees and at the same time get the overall gain equal to 1.
    Since the three RC pairs attenuate the signal from the output of the transistor, the gain of the transistor stage must be equal to the inverse of the attenuation of the three RC pairs, and that comes out to 29 exactly. Usually we would go above this slightly and hope we have some built in non linear gain limiting inherent in the transistor part to keep the output from ramping into saturation. We need that non linear gain limiting because we can never guarantee that we have exactly the right gain in order to sustain oscillation yet not push the output into saturation. If the transistor does not properly provide this function, then we need to add some non linear gain limiting ourselves, usually in the form of a zener or a few diodes to perform a partial clipping action.
     
    Last edited: Jun 20, 2014
  12. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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    You might find this web page helpful:

    http://home.earthlink.net/~doncox/wec/Oscillators.html
     
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  13. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hello again,

    I should have mentioned also that the actual gain of the transistor stage has to have a (theoretical) gain of 29, but that's the AC voltage gain from input to output, so the actual gain may have to be set higher to compensate for other losses such as the loading effect of the emitter base.

    With the RC value set for the frequency of operation, the required gain will always be 29 with a three stage phase shift network.
    RC is equal to 1/(sqrt(6)*w).
     
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  14. Thevenin's Planet

    Thread Starter Active Member

    Nov 14, 2008
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  15. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi again,

    It turns out that calculating the R and C values for the phase shift network isnt hard at all. The gain of the transistor stage is another story though. Good luck with it.
     
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