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  1. LvW

    Phase shift oscillator help

    Regarding your circuit without buffering: Yes - everything correct. The numerator of the loop gain function is imaginary. Hence, also the denominator must be imaginary - in this case the ratio of both is real (requirement for oscillation). Therefore, we set R(denominator)=0 and solve for w.
  2. LvW

    Phase shift oscillator help

    No - for two reasons: 1.) The gain must be somewhat larger than the theoretical value (8). Otherwise, the oscillator would not start safely. 2.) It is not possible to have R2=8R because of tolerances. For R2<8R the circuit will not oscillate and for R2>8R it will safely start - however, with...
  3. LvW

    Phase shift oscillator help

    Hi Ratch - yes, here are the results of my "fresh start:": With reference to your post#14 (calculation in blue): Third line from the bottom: * Left side of the equation is imaginary and right side is complex (imag. plus real). This requires to set the REAL part of the right side equal to...
  4. LvW

    Phase shift oscillator help

    Sorry - I didn`t realize that you were talking about another circuit (unbuffered). Perhaps we should not discuss two different circuits within one thread.
  5. LvW

    Phase shift oscillator help

    Ratch - without any calculation: The loop gain is a product of a frequency-dependent block (3 CR sections) and a gain block, right? Therefore, it is clear that the phase response is determined by the CR-sections only. Hence, the gain A=-R2/R cannot influence the phase function at all. That...
  6. LvW

    Phase shift oscillator help

    As you wrote earlier- "please bear with me" because I needed some time to go through your calculation. But now I can give some comments: No - you were not "wrong", but you didn`t finish the calculation. Let me explain: From the oscillation condition (unity loop gain ) you have derived an...
  7. LvW

    Phase shift oscillator help

    Ratch - the gain of the inverting opamp does not affect the phase shift of the loop gain. We cannot "tune" the oscillator by varying R2. The circuit can continuously oscillate at one frequency only - and that is the frequency where each CR section contributes 60 deg phase shift. "Can you find...
  8. LvW

    Phase shift oscillator help

    Hi Ratch - this is a somewhat "critical" point. Let me explain my view: * The Barkhausen condition (unity loop gain) gives the ideal/theoretical component values - for the frequency determining feedback network as well as the required gain. The corresponding oscillation frequency is calculated...
  9. LvW

    Phase shift oscillator help

    Yes - correct. Good approach starting with 30deg for the denomionator only. Inserting this frequency into the first-order highpass function gives the magnitude of 0.5. I don`t know what to do with your post #23. All the formulas at the bottom are wrong. Any open qestions?
  10. LvW

    Phase shift oscillator help

    My calculation is rather simple: Each CR section contributes 60 deg phase shift at w=wo. At this frequency the loss of this simple HP stage is -6 dB (factor 0.5). We have three decoupled CR sections - hence the total phase shift is - as required - 180 deg and the total loss factor is 0.5^3=1/8...
  11. LvW

    Phase shift oscillator help

    Hi ratch, I must admit your post partly confuses me - and hopefully not also the OP. The first figure shows a block diagram with an amplifier A and a feedback block "beta". And the second figure shows one possible realization with A=-R2/R1. Here "beta" is composed of two buffers and three CR...
  12. LvW

    Phase shift oscillator help

    Yes - the gain of a closed-loop system is A/(1-AB), however, this expression looses its meaning for the case AB=1. Even the mathematics does not allow this case.
  13. LvW

    Phase shift oscillator help

    Writing a gain function requires at first to DEFINE the gain (input and output). In your case it would be: vo/vi. Now - look at you circuit: Vi is directly connected to vo. Therefore, i wrote: It makes no sense to write a closed-loop gain expression for an oscillator circuit.
  14. LvW

    Phase shift oscillator help

    When an amplifier A has a feedback factor B (first drawing) the closed-loop gain is A/(1-AB). As you can see, when the loop gauin T=AB is unity, the denominator is zero - and the closed-loop function approaches infinity. This is the oscillation condition - and because the circuit is oscillating...
  15. LvW

    Phase shift oscillator help

  16. LvW

    Question about B-E voltage.

    Laplace - no technical arguments, but polemic answers only (and I wonder- Why?). Therefore, I reply to one sentence only: Laplace - in post#66 your wrote: "Emitter degeneration is absolutely necessary to reduce the circuit's sensitivity to variations in transistor beta." In my post#67 I have...
  17. LvW

    Question about B-E voltage.

    A newcomer would, of course, ask: So what? Why do we want to reduce the sensitivity to beta? Is this an inportant design parametr? Laplace - can you answer this question and explain how emitter degeneation can reduce this sensitivity (without simply repeating Ic=beta*Ib) ? Please ecxuse me...
  18. LvW

    Question about B-E voltage.

    No role? Did you ever try to analyze (or simulate) a gain stage without emitter degeneration? Again: Vbe plays the major role and determines the main properties of the circuit. And exactly THIS is the reason we are trying to reduce the circuit´s sensitivity to Vbe uncertainties using the concept...
  19. LvW

    Question about B-E voltage.

    Irrelevant? Is it really necessary to explain the role of Vbe again and again? Do you know that the DC value of Vbe solely determines the quiescent current Ic which - in turn - determines the point where the derivative is taken? May I ask you by which means? The result of the process of...
  20. LvW

    Question about B-E voltage.

    Disagree! For designing circuits we do not "predict" Ic because we want to realize a circuit with some specific properties (for example: Voltage gain). And these properties depend on Ic and NOT on Ib. Consequently, we SELECT resp. REQUIRE a certain value for Ic and use the beta value in order to...
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