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Phase Shift Oscillator Question - UPDATE: Circuit is a ring oscillator
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ANTALIFE:
Hello

I am trying to figure out how this phase shift based oscillator is able to well oscillate.

From my understanding for oscillations to occur the phase (between input/output) must have a difference of 360deg or a multiple of.

Knowing that each RC section gives a phase shift of 60deg and each BJT gives a phase shift 180deg, adding these up I get an expected phase shift of 720deg (60+60+60+180+180+180). However when I simulate the circuit in LTspice and try to manually calculate the phase shift I get a value of ~250deg, am I missing something here?

UPDATE: Turns out the circuit is actually a ring oscillator, see orolo's & Hero999's comment
xavier60:
I can't directly answer the question. I notice that R12 is 2K which will make the phase shift of the first stage much less than the others.
spec:
Hi ANTALIFE

I love your question. It is just the sort of thing that I get into a quandary about. :)

I am not going to work it out, because my brain is not functioning yet.

But here are some thoughts to consider.

The phase angle between a transistor(BJT) base and collector is 180 deg, as you rightly say.

But it in not correct to say that the phase angle across a CR network is always 60 degrees: the phase angle varies with frequency.

The capacitor connected between the BJT collector and base, as shown on the schematic, are what is physically fitted. But the vittual capacitance seen by the circuit is the value of the physical capacitor multiplied by the voltage gain (A) of the BJT stage.

A is worked out practically as follows: The value of a BJT internal emitter resistor (re) is 25/ie where re is in Ohms and ie is in mA. And, providing the BJT has a reasonable current gain (hFE), the A of a BJT is Rc/re, Rc being the external collector resistor. For a typical BJT, like a BC227, with an ie of 1ma and an Rc of 1k, A is 1000/25 = 40. For a lower conductance BJT, like a BC54x, the A may be a touch lower, say 30.
bson:
The capacitors bypass base-collector, so are there to limit the bandwidth, which in this circuit effectively sets the oscillation frequency.  If they weren't there the circuit would oscillate around the transistors' effective unity frequency.  They're in parallel with the transistor gain, not in series.  (You'll often see the practice of capacitively coupling input and output of a gain or buffer stage referred to as miller compensation.)
spec:

--- Quote from: bson on November 23, 2018, 11:25:49 am ---The capacitors bypass base-collector.
--- End quote ---
Not sure what you are saying here. How can a capacitor bypass two separate points on a circuit.


--- Quote from: bson on November 23, 2018, 11:25:49 am ---so are there to limit the bandwidth, which in this circuit effectively sets the oscillation frequency.
--- End quote ---
Again not sure what you are saying here. How can limiting the bandwidth set the frequency. The resistor and virtual capacitance set the phase angle which sets the frequency, just like a standard phase shift oscillator. You seem to be thinking about an amplifier where often a miller feedback capacitor is used to roll the gain off to ensure frequency stability. This approach is frequently used in the voltage amplification stage (VAG) of many audio power amplifiers.


--- Quote from: bson on November 23, 2018, 11:25:49 am ---If they weren't there the circuit would oscillate around the transistors' effective unity frequency.
--- End quote ---
Yes, maybe but cant see the relevance of this statement.


--- Quote from: bson on November 23, 2018, 11:25:49 am ---They're in parallel with the transistor gain, not in series. 
--- End quote ---

Afraid this does not make sense. How can anything be in parallel with gain- the dimensions do not match.


--- Quote from: bson on November 23, 2018, 11:25:49 am --- (You'll often see the practice of capacitively coupling input and output of a gain or buffer stage referred to as miller compensation.)
--- End quote ---

Yes, and Miller compensation uses the Miller effect as I describe.
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