RC Phase Shift Oscillator not Oscillating

[Cup of Joe]

The only thing is that my DMM is brand new. I bought it quite recently. So I don't see how it could be faulty right off the bat. Although, I could be wrong and the DMM has a defect from the factory. BTW, my DMM is a Proster, model VC99 in case you are wondering.

 

[Cup of Joe]

If you test your DMM with the frequency of AC mains, does it show it correctly?

Yes, I just tested it, it works. It shows 123 VAC and 60 Hz. So the problem must be my circuit. I will add that extra transistor on the previous page for better gain.

 

[Cup of Joe]

Just to be sure, the output of the oscillator has to be connected to ground or -5V in my case, right?

 

[Cup of Joe]

So I added that extra transistor, but I am getting inconsistent results. One time I got a max AC voltage of 3.3 and then got a max of 1.5 later. The minimum AC voltage is about 0.23. Also, I seem to be having the same issue as before: the frequency measurement on my DMM seems to be fluctuating in the kHz range still. The gain should be much higher than before, and I see a higher voltage at the output because I added another BJT.

There seem to be many overlapping harmonics at the output. Maybe my BJTs are producing lots of noise?

 

[Delta2]

So I added that extra transistor, but I am getting inconsistent results. One time I got a max AC voltage of 3.3 and then got a max of 1.5 later. The minimum AC voltage is about 0.23. Also, I seem to be having the same issue as before: the frequency measurement on my DMM seems to be fluctuating in the kHz range still. The gain should be much higher than before, and I see a higher voltage at the output because I added another BJT.

There seem to be many overlapping harmonics at the output. Maybe my BJTs are producing lots of noise?

if you try only with the new transistor what do you get? Same things as before? (frequency at DMM to be Khz, 0.5V peak and such?)

 

[Baluncore]

I think you have painted yourself into a corner with your selection of component values and frequency. When I simulate the oscillator with very low value resistors it needs at least Ce = 1000 uF before it will oscillate with a 2N2222.

Here are LoZ and HiZ versions that do oscillate at about 20 Hz with a single 2N2222.
Notice the value of Ce required, if I reduce it, the oscillation will not start.
Lo-Z, big capacitors.

Hi_Z, small capacitors.

 

[tech99]

Thank you Baluncore for these successful circuits. In the second circuit, how do we account for the low input impedance of the transistor shunted across R3, which will lower the effective value?

 

[Baluncore]

Base bias R3 and R4 are equivalent to about 33k. The base is not a particularly low impedance since collector current is about 100 uA, so base current is less than 1 uA, average 300 nA.

 

[Cup of Joe]

@Baluncore Can you please explain how to know what value of Ce (emitter cap) to use when given a high or low Z for the feedback network?

I only have 100 uF, 10 uF, 22 pF, and 10 pF caps. I would have to buy some caps if I cannot get my circuit working with these values.

 

[Cup of Joe]

New idea: can someone please show an example of an RC phase shift oscillator at whatever frequency you choose? I would like to learn from your example. The circuit that Baluncore made I cannot do because I do not have those part values. Here are my component values:

- Caps: 100 uF, 10 uF, 22 pF, 10 pF
- Resistors: 10, 100, 220, 330, 1k, 2k, 5k, 10k, 100k, 1M
- My BJT: https://www.onsemi.com/pdf/datasheet/pn2222-d.pdf

Thank you in advance.

 

[DaveE]

New idea: can someone please show an example of an RC phase shift oscillator at whatever frequency you choose? I would like to learn from your example. The circuit that Baluncore made I cannot do because I do not have those part values. Here are my component values:

- Caps: 100 uF, 10 uF, 22 pF, 10 pF
- Resistors: 10, 100, 220, 330, 1k, 2k, 5k, 10k, 100k, 1M
- My BJT: https://www.onsemi.com/pdf/datasheet/pn2222-d.pdf

Thank you in advance.

If you want to build analog circuits like this you will need to buy some capacitors to fill in your gaps. 0.1uF and 0.001uF at a minimum. You can buy them now, or struggle for a while and then buy them later. Hobby websites like jameco.com and sparkfun.com have component assortments that aren't too expensive.

 

[Baluncore]

Can you please explain how to know what value of Ce (emitter cap) to use when given a high or low Z for the feedback network?

If the simulation does not oscillate, then increase the value of Ce by a factor of 10. The transistor amplifier AC gain rises with the emitter capacitance. The Re and Rc set the DC bias point of the transistor. You picked a difficult circuit for your first attempt.

Rather than building up circuits from real parts you could download a free copy of LTspice.
https://en.wikipedia.org/wiki/LTspice
That way you could try any component with any value you want.