Hello everyone. This is my first post here. I am an aircraft electrical systems instructor and the facility I teach at has a course for the NCATT certification that I am going to be teaching. I have the license, but the majority of my experience and knowledge is to the systems, not the internal circuitry of the components. The only part that I'm going to be teaching that I am struggling with is two of the four types of oscillators.

We talk about the Armstrong, Hartley, Colpitts, and crystal-controlled Pierce oscillators. The reference material I have from the old NCATT website (really, it's just older Air Force training documents) only goes into detail about the Hartley and Pierce. While trying to get more info I've realize that the diagrams I have for the Armstrong and Colpitts are not found anywhere on the internet that I have been able to access. I understand the concepts and for the most part what each component is doing, but my biggest issue is signal/current flow.

I'm attaching the images for the Armstrong and Colpitts oscillators. Could someone help to explain how the BJT is being biased and the current flow (by the way, NCATT teaches electron flow, negative to positive, so that would be preferred) as it pertains to initial power input, the feedback network, and output signals.

I really appreciate any help I can get.

 

hi nb,
There is a link missing between the junction of R2/3 and L2 on the Armstrong diagram.

http://www.electronics-tutorials.ws/oscillator/colpitts.html
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EDIT:
Also check this out.
https://www.elprocus.com/armstrong-oscillator-circuit-working-application/

 

Thank you, that makes a lot of sense and really clears up the Armstrong.

 

hi,
As you are teaching, I would consider learning the LTSpice free download simulator, you could show your students the waveforms etc for a great number of circuits.
Note my Edit for the prev post
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I understand "how" the circuits work, and really with both of the circuits the issue is with the particular drawings. I see signal flow path much better with the added connection on the Armstrong. And with the Colpitts, I'm not finding any drawings that look remotely like the one I have. I've been told to remove the connection for R1 from the base to ground, the entire line, and move R1 so that it is connected in series with the collector of Q1, which makes more sense to me. I just don't see the signal flow in my head when looking at it.

Essentially, with the Colpitts, I'm trying to see the AC signal path vs. the DC signal path.

 

hi,
Where did you get the original drawings, they are a bit of a mess, missing links etc.
Why don't you analyse a web Colpitts tutorial circuit.?

https://www.elprocus.com/colpitts-oscillator-circuit-working-and-applications/

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Those drawing are the ones in our course material, and specifically used as figures on the NCATT certification test, which is why I'm focusing on them.