Help me understand the Micro Vibe

[jatalahd]

How can come 180^o phase shift between 2 IN PHASE signals without anything to lead/lag them..??

Please forgive me Antonis

This is the reason I presented the hybrid-pi model, because it is a "standard" model and it shows that the 180 degree shift is obtained by taking the signal to the feedback loop from ground potential and the emitter of the transistor is the ground for the "input" part of the feedback loop. This is the same as "inverting" the signal, multiplying by -1, or applying 180 degree shift. But now I am not sure if we are even talking about the same thing.

I just hoped that my drawing would have been enough

[antonis]

But now I am not sure if we are even talking about the same thing.

Definately not...

[jlo]

Emitter followers (Darlington or not) are not famous enough for phase shifting..
(maybe that's the reason for also called "Voltage followers"..)

But the feedback is 180 shifted?

Should we remane topic "Endlessness is not enough"..??
How can come 180^o phase shift between 2 IN PHASE signals without anything to lead/lag them..??

Where's the dead horse emoji?  I'm sure you've had enough of me.  Just going by what was posted earlier:  "The emitter-follower circuit (with only bias and emitter resistor) is already a series-series feedback amplifier on its own, where the FEEDBACK voltage is 180 degrees out of phase from the OUTPUT. See the diagram below:"

[antonis]

You're free to go anywhere might be convenient to your understanding..

As I see it, Emitter follower circuit represents Voltage-series feedback topology with 100% negative feedback..
(entire output voltage is fed back in series with input..)

As Paul already told you, the issue here is (less than unity) gain rather than phase shift..

[jlo]

Quote from JC I found from another thread
" The Univibe oscillator belongs to the phase-shift oscillator family - where a combination of greater than unity loop-gain and at least 180degree phase shift in the loop is required for the circuit to start and maintain itself ... each cap produces 90deg of shift so three caps gives a total maximum potential of 270deg ... this kind of principle is used in Fender amp Tremolo oscillators for example - except in the Vibe case the voltage gain of a Darligton follower is less than unity, so the oscillator loop must be operating in the current variable - with a (current) gain of Beta^2 - and capacitors providing phase shift in the same way they would in a voltage-mode oscillator ... in theory phase-shift oscillators exhibit exponentially increasing Amplitudes with time and so require amplitude limition to keep the output signal steady, hence the two diodes clamping the middle shifting cap ... using LED's or Germanium diodes will alter the output size indirectly ..."

I thought you needed a 360 shift for oscillation to occur?

[jlo]

But you just need enough shift so that the loop gain is sufficient to cause oscillation?

[PRR]

The usual amplifier in the Phase-Shift is an inverter. We need another inversion in the network.

[antonis]

But you just need enough shift so that the loop gain is sufficient to cause oscillation?

https://en.wikipedia.org/wiki/Electronic_oscillator
https://www.youtube.com/watch?v=XVS8Puf4tiw
https://www.youtube.com/watch?v=aJAZHPqEUKU
https://www.electrical4u.com/what-is-an-oscillator/
https://learnabout-electronics.org/Oscillators/osc10.php
https://electronics.howstuffworks.com/oscillator.htm
https://www.elprocus.com/different-types-of-oscillator-circuits-its-applications/

[jlo]

"As any single high pass filter can produce a phase change of up to 90°, it would seem that, in theory, only two such networks, would be needed. However, using two filters with each producing a 90°phase shift would mean that, as the phase graph in Fig. 3.1.1 shows, the phase response curve is quite flat at and above 90°, so any drift in frequency would have little effect on the 180° phase shift produced. This would mean that if the frequency of the oscillator changes, due to a change in temperature for example, there would be hardly any change in the amount of phase shift, so frequency stability would be poor. It can be seen from Fig. 3.1.1 that at 60° or 45° the phase response curve is much steeper, and so with three filters producing 60° each, or four filters providing 45° each, to make up the required 180°, frequency stability will be much better."

From https://www.learnabout-electronics.org/Downloads/Oscillators-module-03.pdf

[jlo]

Can you please explain the RC network and the dual pot?   And how do you get the additional gain?

Yes.

As a pre-requisite you need to understand the RC phase-shift oscillator in its more common form (feedback from collector to RC stage). This infomation is available in the internet (just search by "RC phase-shift oscillator"). The following information presented here is top secret and will be buried in this thread and never found again

There are four different feedback topologies in amplifier design: series-shunt, shunt-series, shunt-shunt, series-series (use internet search if you want to know more). The "normal" RC phase-shift oscillator uses the shunt-shunt feedback model. The emitter-follower circuit (with only bias and emitter resistor) is already a series-series feedback amplifier on its own, where the FEEDBACK voltage is 180 degrees out of phase from the OUTPUT. See the diagram below:



In this image, we have the signal source added and the feedback voltage Vf is summed as "inverted" to the input. Please ignore the RC/RL, since this model can be applied to the common-emitter amp as well but not now. Hopefully this is clear so far. In the oscillator, the signal source can be removed, since the input signal is taken 100% as a feedback signal from the output.

Next we draw the Uni-Vibe RC network on top of the same feedback diagram (NOTE: redrawing circuits in standard form is a huge help to understand them)



Now it is starting to look as the "normal" RC phase-shift oscillator. So we have 180 degree shift already at the input side of RE and another 180 shift from the RC network. This sums up either as 180 - 180 = 0 OR 180 + 180 = 360 = 0 at the frequency of oscillation. When there is positive feedback (0 degree shift) the thing will oscillate. But wait...

There is also a theory called the "Barkhausen stability criterion". It says that for oscillation to happen, the LOOP GAIN of the circuit must be equal or larger than -1. There is so much false information floating around saying that the gain of the circuit must be so-and-so much for oscillations to happen (to make up the loss of the RC feedback network). This is not so. The basic emitter follower circuit can easily have a LOOP GAIN of over 100 (although the voltage gain is ALWAYS less than 1), and there is no magic related to this. The loop gain is calculated differently than "normal" gain. So in this case the Barkhousen stability criterion is fullfilled and also overly ensured by the darlington's high current gain (these days a single high-Beta tranny like BC549C would be enough here).

By adjusting the resistances, you adjust the cut-off frequencies of the RC-pairs, thereby affecting the oscillation frequency. Not any different than in the normal RC phase-shift oscillator.

And if your next question is how to calculate the loop gain for this specific circuit, then I am sorry to say but you are not ready for it. I did it using numerical math tools and utilizing matrix algebra. There is no simple equation to give.

Ok I think I understand now.  The diagrams were very helpful.  The voltage from the collector in an emitter follower is 180 out of phase with the input.  The RC network flips it 180 back into phase and results in oscillation when fed back with the input.  Is that correct?   

[antonis]

The voltage from the collector in an emitter follower is 180 out of phase with the input.  The RC network flips it 180 back into phase and results in oscillation when fed back with the input.  Is that correct?

NO..!!!

There isn't any RC circuit connecting Collector to anywhere so there isn't neither feedback from Collector nor any phase shift..

[duck_arse]

an emitter follower ....

[jatalahd]

Ok I think I understand now.  The diagrams were very helpful.  The voltage from the collector in an emitter follower is 180 out of phase with the input.  The RC network flips it 180 back into phase and results in oscillation when fed back with the input.  Is that correct?   

The 180 degree phase-shift from the emitter-follower amplifier stage is here due to using the emitter as ground potential and ground as "signal". That makes the amplifier an inverting amp, you simply switch + and - potentials to invert the signal to the feedback loop. This you can do for any circuit that carries a sine wave. By measuring the signal between + and - using - as ground you see the "in-phase" signal. By measuring the signal between + and - using + as ground you see the same signal, but inverted (out of phase by 180).

After this "trick", the rest is generic RC phase-shift oscillator theory, which you can find using the links that Antonis kindly provided. Without the diodes connected, there is absolutely nothing special in this circuit. It is a plain old RC phase shift oscillator and all the general theories related to it hold also in this circuit.

[antonis]

The 180 degree phase-shift from the emitter-follower amplifier stage is here due to using the emitter as ground potential and ground as "signal". That makes the amplifier an inverting amp, you simply switch + and - potentials to invert the signal to the feedback loop. This you can do for any circuit that carries a sine wave. By measuring the signal between + and - using - as ground you see the "in-phase" signal. By measuring the signal between + and - using + as ground you see the same signal, but inverted (out of phase by 180).

It'a pitty that nobody thought this very dealing for an CE amp to get both amplified signal and IN PHASE... 

To be more serious, Jarmo:
True and Correct all the above but let's stay on a more conventional acceptance about potential difference points definition..
(or else, we might be seriously accused for OP committed suicide complicity..)

[jlo]

Ok I think I understand now.  The diagrams were very helpful.  The voltage from the collector in an emitter follower is 180 out of phase with the input.  The RC network flips it 180 back into phase and results in oscillation when fed back with the input.  Is that correct?   

The 180 degree phase-shift from the emitter-follower amplifier stage is here due to using the emitter as ground potential and ground as "signal". That makes the amplifier an inverting amp, you simply switch + and - potentials to invert the signal to the feedback loop. This you can do for any circuit that carries a sine wave. By measuring the signal between + and - using - as ground you see the "in-phase" signal. By measuring the signal between + and - using + as ground you see the same signal, but inverted (out of phase by 180).

After this "trick", the rest is generic RC phase-shift oscillator theory, which you can find using the links that Antonis kindly provided. Without the diodes connected, there is absolutely nothing special in this circuit. It is a plain old RC phase shift oscillator and all the general theories related to it hold also in this circuit.

So is there some semantics involved in terms of whether a phase shift happens or not? 

[PRR]

"As any single high pass filter can produce a phase change of up to 90°....

Wrong. "up to almost 90°". Or "90° at infinity, which is out of reach."

[jatalahd]

So is there some semantics involved in terms of whether a phase shift happens or not? 

Regarding the phase shift in the amplifier there is no special semantics involved, if an emitter resistor exists without a parallel bypass capacitor, there is always negative feedback from output to input for AC-signals. Negative feedback through emitter resistor is present both in an emitter follower and a CE amplifier, if the emitter resistor is not bypassed for AC-signals.

May I ask why is it so important for you to understand this oscillator thing?

[jlo]

So is there some semantics involved in terms of whether a phase shift happens or not? 

Regarding the phase shift in the amplifier there is no special semantics involved, if an emitter resistor exists without a parallel bypass capacitor, there is always negative feedback from output to input for AC-signals. Negative feedback through emitter resistor is present both in an emitter follower and a CE amplifier, if the emitter resistor is not bypassed for AC-signals.

May I ask why is it so important for you to understand this oscillator thing?

Its not that important.  And I appreciate everyone's help and patience.  I fell down a rabbit hole trying to understand it but most likely because of my lack of knowledge I was getting confused with a lot of the posts.   This had me going in circles.  I read up and understood the RC phase shift oscillator concept.  But didn't understand the implementation in the UV LFO because of the use of emitter follower vs common emitter.   In my mind I was getting contradictory information.   Time for all of us to move on!   I have more questions but lets put this phase shift stuff to bed.

[PRR]

> emitter follower vs common emitter

It's common emitter. Just connected funny.

[jlo]

Thx for everone's help so far.  How about C13 on the schematic.  Its a 33uF cap

This is for decoupling?  How is that value chosen?  If I want to run the pedal at 18V and use a 40mA bulb and opamps with a higher current draw, will I need to increase this?