No oscillations?

[chopnhack]

Hello all, I am trying to learn about oscillators as part of this project. I have built the first circuit, pictured below, multiple times, but I do not measure any frequency across the output and ground. I then tried to model it in LTSpice, but it does not oscillate there either. Is there anything obvious that I might be missing? The source for the circuit has been a reputable one in the past for other subjects. I also tried the second circuit pictured below with no success. The capacitor values were changed to what I had available. I tried .uic mode as well as the ramp up mode to no avail. Thank you in advance for any input.

 

[danadak]

First and foremost the bias network, R4, is swamped by
R3 value, its effectively not in circuit, and bias is screwed
up.

Look at DC bias values, especially Vce, should be at least
2V to get your Q1 in active region. I would do this with
R3 set to infinity.

Second C4 should be connected to Q1 collector, not the
positive supply. The way it is now there is no fdbk/phase shift
back thru phase shift sections into base of Q1 to oscillate.

 

[chopnhack]

danadak If I followed you correctly, the new schematic looks very similar to the second one of the original post.

I realized that I had incorrectly copied the schematic. The above is a better attempt at the video's schematic with your suggestions. I increased R3 to 100Meg ohm and reduced the voltage divider R5/4 to 2 ohm each which allowed ~2.3v potential across the base to ground, I then realized that you mentioned Vce. Vce was no where close to that so I played with R6's value. After a few iterations, I did not get any oscillation. I tried the second schematic as well:

In the second image, there is about 2.4V across the collector and emitter. Again, no oscillation. Any thoughts? Thanks again.

 

[Diver300]

Have you tried the circuit in real life, or is it just a simulation?

In real life, very small amounts of noise from thermal effects or pickup from electrical device will cause a small disturbance which is then amplified and filtered, and then amplified more. The result is that the oscillation will build up and get larger and larger until the amplifier hits its limits.

You might be able to get the simulator to oscillate by trying different start values at each capacitor.

 

[chopnhack]

Have you tried the circuit in real life, or is it just a simulation?

Only the circuit from the original post. Got tired of ripping it up and starting over several times today

 

[Karklebapper]

I am not an expert on oscillators but I have played around with them. One issue is that your RC network has the same impedance going in as coming out. You really want more of a one way signal flow but the loading of each stage greatly affects the phase shift. With all of your RC stages, there may be too much attenuation around the loop even if the impedances are fixed.

Of course it is easy to relax with a "relaxation" oscillator using an op amp or comparator.

Another thing that works: Start with a logic inverter like 74HC04 or 74LS04.. Wind a bunch of wire turns on a pencil and connect that coil from input to output. Connect a capacitor (maybe 1 nF) from output to ground. Connect a 3x or 4x larger capacitor (3.3 nF) from input to ground. Bypass the power supply with a proper bypass capacitor and power up. This should oscillate until power is removed or the chip fails. (This circuit may overstress the chip so this design is not recommended for any real world circuit design.)

 

[danadak]

The basic phase shift network needs to look like your original post,
now you have placed the first stage with 2,2K in parallel (in effect)
with the 100 ohm in Q1 collector.

The 100 ohm in the collector kills the gain its so low. A CE stage the low freq G =~
gm x Rcollector, and gm is ~

I would advise you work on getting bias correct, tons of papers on net to do that,
with a collector R somewhere in the few single digit K range. Bias R's in the 10's of K
even 100K range.

 

[chopnhack]

danadak Thank you for the follow up. I found a tutorial on transistor biasing. When I am through with it, I will try again. It's clear as mud right now

 

[JimB]

The oscillator circuit which you are trying to build is known as a Phase Shift Oscillator (PSO)
A few months ago I did some work on a PSO circuit for a talk which I was to present to the local amateur radio society.

The upshot is that the PSO very nearly drove me mad, it was very difficult to make it work reliably by just chopping and hacking component values.

To get you started, here is a circuit which does work:

It may look a bit gaudy, it is cut from a PowerPoint file which I created for the talk.

Resistor R5 sets the gain of the amplifier stage, in my demo model, R5 was 39 Ohms.
You can start with a lower value to be sure that the oscillator starts, and then adjust R5 to give a nice sinewave output.

I note that in another post you are looking to make an oscillator to run in the range 50 to 150MHz,
if that is the case a PSO is most definitely NOT the way to do it.

JimB

 

[Karklebapper]

Here is one that also works. I just threw this together quickly and cannot say much about it, other than it oscillates. But...note that it does not oscillate until V2 "kicks" it into oscillation. That is one aspect in which simulation differs from hardware: With hardware, there is always some noise to get it started. But the simulation is likely to have no noise.

 

[chopnhack]

JimB - Wow, thank you sir!

It did drive me mad today... I don't think I would have gotten it to work correctly anyway, I was dealing with too low of a source (5VDC). In LTSpice, 5V doesn't allow it to oscillate, but at 10V it works.

The frequency looks to be about 900Hz - 1/0.001sec - about 1 ms per completed cycle if I read it correctly.
And you are correct, this would not be way moving forward! I was trying to get some hands on experience in generating a signal to then read into my mcu. I don't have any crystal oscillators on hand, which is the next thing I wanted to experiment with.

Thank you gents! Really appreciate the information.

 

[danadak]

The "kick" can just be the power applied at t > 0

Using FFT this oscillated ~ 138 Hz.


Regards, Dana.

 

[Karklebapper]

1) You are using a different simulator. That seems like 1 possible explanation.

2) My DC source does "start" after T=0. But my circuit appears to be more "symmetrical" and perhaps it did not inject any "net" perturbation.

 

[Tony Stewart]

The gain is less than the cascade loss.

Grow your hair back with a Darlington or even better a CMOS Op Amp R2R

 

[Karklebapper]

Grow your hair back with a Darlington or even better a CMOS Op Amp R2R

Oh, my Darlington! I like comparators for oscillators; at least for driving logic. An open collector (such as LM339) output can be followed by a CMOS inverter to get active pullup/pulldown. But this is a square wave output; not sinewave.

 

[danadak]

Plenty of G around osc freq, eg. phase 0 degrees, (my prior sim 138 Hz osc freq)

Regards, Dana.