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.)
Thanks for the input everyone, looks like I will have to do a bit more digging.
Also as pointed out I should have said the RC sections give a phase difference of 60deg at a given frequency, and in the LTspice sims this frequency is 59mHz
Thanks for the input everyone, looks like I will have to do a bit more digging.
Also as pointed out I should have said the RC sections give a phase difference of 60deg at a given frequency, and in the LTspice sims this frequency is 59mHzDid you intend R12 to be 2k?
Not sure what you are saying here. How can a capacitor bypass two separate points on a circuit.
This is not a phase shift oscillator. It's more like the classic LED chaser circut, a ring of switches that trigger each other. When Q4 is saturated, Q6 must be off to keep Q4's base high. Since Q4 is saturated, its collector is low, which will turn Q5 off. As Q5 turns off, its collector goes up, which then turns Q6 on. As Q6 turns on and goes into saturation, Q4 is turning off.
So the thing goes:
Q4(sat), Q5(turn off), Q6(turn on) -> Q4(turn off) Q5(turn on) Q6(sat) -> Q4(turn on) Q5(sat) Q6(turn off) -> Q4(sat) Q5(turn off) Q6(turn on) -> etc.
If your circuit is symmetric, you should see identical waveforms in the transistor collectors, 120 degrees out of phase.
This is not a phase shift oscillator. It's more like the classic LED chaser circut, a ring of switches that trigger each other. When Q4 is saturated, Q6 must be off to keep Q4's base high. Since Q4 is saturated, its collector is low, which will turn Q5 off. As Q5 turns off, its collector goes up, which then turns Q6 on. As Q6 turns on and goes into saturation, Q4 is turning off.
So the thing goes:
Q4(sat), Q5(turn off), Q6(turn on) -> Q4(turn off) Q5(turn on) Q6(sat) -> Q4(turn on) Q5(sat) Q6(turn off) -> Q4(sat) Q5(turn off) Q6(turn on) -> etc.
If your circuit is symmetric, you should see identical waveforms in the transistor collectors, 120 degrees out of phase.
@ ANTALIFE
What frequency is the circuit actually oscillating at? I can't make out the legend on the scope screen. Also what supply voltage does the circuit have.
My back of cigarette packet calculation shows around 500Hz.
This is not a phase shift oscillator. It's more like the classic LED chaser circut, a ring of switches that trigger each other. When Q4 is saturated, Q6 must be off to keep Q4's base high. Since Q4 is saturated, its collector is low, which will turn Q5 off. As Q5 turns off, its collector goes up, which then turns Q6 on. As Q6 turns on and goes into saturation, Q4 is turning off.
So the thing goes:
Q4(sat), Q5(turn off), Q6(turn on) -> Q4(turn off) Q5(turn on) Q6(sat) -> Q4(turn on) Q5(sat) Q6(turn off) -> Q4(sat) Q5(turn off) Q6(turn on) -> etc.
If your circuit is symmetric, you should see identical waveforms in the transistor collectors, 120 degrees out of phase.
You could have realized that earlier when it was pointed out that "R12 is 2K which will make the phase shift of the first stage much less than the others"