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I have a circuit like this:

enter image description here

Given a low or a high impedance feedback network, how do I choose the right emitter cap value Ce? I know that the emitter cap depends on the impedance of the feedback network, but I am lost when it comes to finding what value it needs to be.

What are the rules of thumb? What are the standards?

Also, if you could explain what emitter resistor value I need as well for a given feedback impedance, that would be great.

The BJT I am using is the PN2222.

Please know that I never really built an oscillator before and I might be asking something simple that might seem obvious, but not to me.

Please note: I am trying to learn how to build an RC oscillator as a hobby project, for the sake of learning how to build oscillators. I do not want to have an RC IC, but I want to build one myself.

Here is my circuit:

enter image description here

Null
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Shocked
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  • What are the values of C and R in your schematic, if you guesstimate the Miller effect, are we solidly above that? Why the three RC combinations in series, that makes not that much sense to me, unless you want to maximize dependence on component tolerances, and make calculating stuff harder! – Marcus Müller Jul 25 '21 at 19:14
  • I don't think there's a rule of thumb here: you'd calculate an equivalent schematic where you combine the two right C and the three R to a single complex impedance, then solve analytically or numerically. – Marcus Müller Jul 25 '21 at 19:16
  • @MarcusMüller I added my circuit, please take a look. I am unsure about the values of the components though. – Shocked Jul 25 '21 at 19:28
  • these values make no sense at all to me; the uncertainty in your 100 µF alone should have told your that something in the order of 22 pF will not work. Again, why do you have these three RC elements? What's the motivation for this circuit? I mean, it didn't fall from the skies, you've designed it, you must have had a reason. – Marcus Müller Jul 25 '21 at 19:30
  • @MarcusMüller Why won't it work? Is it because the caps are too small? – Shocked Jul 25 '21 at 19:32
  • counterquestion: why *should* it work? It's your circuit, you must at least have gotten the idea somehow!I mean, I see you want to build some oscillator, but you still haven't been able to explain why the three R and three C. THese make things more complicated. Why? – Marcus Müller Jul 25 '21 at 19:33
  • Where do their values come from? If you actually don't know how to build this kind of oscillator, that's OK, but then ask "How do I build this kind of oscillator, my approach is..." instead of "Please tell me how my very strange circuit with random values can be fixed" without even stating a design frequency. – Marcus Müller Jul 25 '21 at 19:41
  • @MarcusMüller I updated my question stating why I am building it. Also, I do not understand your abrasiveness towards my learning and development. I am brand new to this and making mistakes is how you learn. So excuse me for my ignorance. EDIT: I posted this after you wrote the following comments. Thank you for sending them. – Shocked Jul 25 '21 at 19:42
  • I don't mean to be abrasive. Still, could you tell me at least why you have the three R and three C? Why did that come from? – Marcus Müller Jul 25 '21 at 19:43
  • @MarcusMüller https://www.electrical4u.com/rc-phase-shift-oscillator/ – Shocked Jul 25 '21 at 19:44
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    ... please link your sources in the question right from the start next time. – Marcus Müller Jul 25 '21 at 19:44
  • Let us [continue this discussion in chat](https://chat.stackexchange.com/rooms/127904/discussion-between-shocked-and-marcus-muller). – Shocked Jul 25 '21 at 19:45
  • anyway, that text is wrong in implying that the three stages of CR filters can be considered as three independent filters; that's simply not true! With that in mind, you should simply find a simpler circuit built on less wrong assumptions. I think fixing this circuit here makes little sense. – Marcus Müller Jul 25 '21 at 19:46
  • @MarcusMüller I have sent a link in the chat, please have a look. – Shocked Jul 25 '21 at 19:50
  • @Shocked Also see [some technical analysis about required gain as well as computing frequency](https://electronics.stackexchange.com/a/371292/38098) and [some meandering on the topic, but maybe helpful](https://electronics.stackexchange.com/a/337900/38098). – jonk Jul 26 '21 at 06:39

1 Answers1

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  • This phase shift oscillator needs to have impedance low enough to drive Rbe and 50k is far too high.

  • Vbe must be driven with at least 50mV around 600mV to oscillate and enough Collector current to increase the gain to overcome the 3rd order attention.

So let’s lower the impedance and get rid of useless ReCe and switch to negative feedback bias instead of H bias. Using +/-5V gets you close to 0V average.

Design result here.

Although reducing Rcb negative feedback attenuates the input, it also provides more base to collector current necessary to get the open loop gain you need. 1M pullup also provides some assistance to get more Av from Ic by making the Rbe smaller. enter image description here

Tony Stewart EE75
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    With a few component changes one can get 1MHz with 10% https://tinyurl.com/ygvmnyx5 – Tony Stewart EE75 Jul 25 '21 at 20:55
  • I just order the required components since I don't have them. – Shocked Jul 25 '21 at 21:35
  • In your first simulation, I see that you aren't getting a negative voltage swing, but it swings in the positive region. How can that produce a valid oscillation? I thought there has to be a positive and a negative swing in order for there to be oscillation? – Shocked Jul 25 '21 at 21:40
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    The offset is from Vbe and Ib. It works also changing Vee from -5 to 0 V (single supply) then slightly higher than 5V/2 avg. by adjust Rcb to 100k and Rbv+ to 1M notice to get 180 deg phase on FB , C does not need to exactly equal. https://tinyurl.com/ydmw3ayh. This is not nearly as stable as a xtal osc. By about 10k times – Tony Stewart EE75 Jul 25 '21 at 21:59