Understanding Colpitts Oscillator

[srobertjames]

I understand the basic principle of an oscillator (amplified feedback + filter). But I can't understand the workings of any of the standard ones, such as the Colpitts oscillator.

I'd like to understand how it works well enough that I can design my own, understand the variants, and predict behavior. I've found plenty of sites which show you an example and perhaps do some hand waving, but none which actually explains how it works.

 

[QuietMan]

Wikipedia has a pretty decent entry.

Colpitts oscillator - Wikipedia, the free encyclopedia

I'm assuming you understand where the inverted feedback is coming from?

 

[srobertjames]

I'm assuming you understand where the inverted feedback is coming from?

No, I don't... Could you explain?
I couldn't figure out the wikipedia article, either.

 

[QuietMan]

You are grounding the middle of a tank circuit, between two capacitors. This is the reference. The tank circuit doesn't care, but from the vantage of the ground the signal on each end of the tank circuit is inverted from the other.

Since a standard common emitter transistor inverts the signal 180° this is a very useful property.

 

[RadioRon]

This book can be helpful:
"Crystal Oscillator Circuits, revised edition" Robert J Matthys, Krieger Publishing Co., Malabar Florida.

 

[srobertjames]

Thank you - that helps a lot. I think my problem is that I have only a superficial understanding of tank and similar circuits... Horowitz & Hill have very little about inductors, and I believe nothing about resonance or tanks.

I've read a bit about them, but I think I need to deepen my understanding. It's not at all obvious to me why splitting the tank in the middle will yield two inverted signals... Can you elaborate on how that works, and perhaps direct me to a good resource to deepen
my understanding?

(I worked through the experiments in an intro. analog circuits course, but, like I said, they didn't touch RLC or even L so much... if there's a good course in the same vein I could work through, I'd be very appreciative.)

 

[QuietMan]

It's not so much about splitting the signals, it more a matter of reference. If I have a battery, and I choose a voltage between the poles of the battery, it appears I have a plus and minus voltage. The battery is still what it is. It is the choice of reference that is different.

 

[colin55]

One of the things you are not aware of is this:

If you deliver a voltage (actually a current) to a coil and then remove the supply, the magnetic field produced by the current will collapse and produce a voltage of the opposite polarity.
This is the main point you are missing.
All the circuits you are dealing with have a coil and it produces a voltage of the opposite polarity at some point in the cycle.
When you take this into consideration, you will understand how this "reverse" or "opposite" or "negative" voltage causes the transistor to turn on more.

The second point you may not be aware of is this:

This applies to circuits with a transformer.

When you apply voltage (current) to the primary of a transformer, a voltage is produced in the secondary. This is due to expanding magnetic flux. But after a short period of time the current will still be flowing but the current will not be increasing and thus the magnetic flux may be a maximum but it will not be increasing and thus no voltage will be produced in the secondary.
This is one of the other secrets of how an oscillator works.

 

[QuietMan]

A tank circuit rings, but it is not related to a transformer. The ringing (resonance) is what causes the oscillation, but colpitts is about the form of feedback. Almost all the oscillators named after someone have a unique form of feedback, and most of them have tank circuits.

 

[colin55]

A tank circuit rings

This is not true. A tank circuit does not “ring” because it has a capacitor that prevents this.
Ringing is a phenomenon that is an unknown, uncontrolled, maybe unwanted, high frequency oscillation. A tank circuit, by the mere fact of its name, is a circuit that oscillates at a known, wanted, reliable frequency.
Ringing is: “strictly-speaking,” not resonance.
Again, saying ringing causes the oscillation is not accurate. You should say: oscillation causes “ringing.”

But most important, I want to get a reply from srobertjames.
I did not say a Colpitts oscillator involves transformer-action. I merely wanted to point out the two mysteries of an inductor, as you completely skirted around this when describing the action of feedback.