Am I going crazy???

[Mr J]

Hi everyone,

I was just going over some class materials for the Boyscout Radio Merit Badge and found an oscillator schematic on page 32 that looks like it will not work. I am going crazy or is there no feedback loop to keep the oscillator going?

[Whales]

Hmm.

Current entering the base will pull the top of the crystal down.  Then eventually the voltage at the top of the 10K resistor drops, prohibiting the transistor from conducting.  After the inductor collapses the top of the 10K resistor charges up again, also pulling the top of the crystal and base up.  Repeat.

[Mr J]

here is a close one that I breadboard and does work, look like the one on the top is missing a cap between Base and Emitter?

[Andy Watson]

Capacitance between the collector and base provide the feedback.

[JimRemington]

here is a close one that I breadboard and does work

Try breadboarding the original. I'll bet it works too.

[PA0PBZ]

here is a close one that I breadboard and does work

Try breadboarding the original. I'll bet it works too.

The breadboard will take care of the feedback for you.

[StillTrying]

Capacitance between the collector and base provide the feedback.

Is that positive or negative feedback?

[Andy Watson]

Capacitance between the collector and base provide the feedback.

Is that positive or negative feedback?

"Feedback" was probably a poor choice of word. The collector-base capacitance couples the output tuned-circuit back to the input tuned-circuit, i.e. the crystal. Try searching for "tuned plate tuned grid oscillator".

[Cyberdragon]

It's a Miller crystal oscillator.

[StillTrying]

"Feedback" was probably a poor choice of word. The collector-base capacitance couples the output tuned-circuit back to the input tuned-circuit, i.e. the crystal. Try searching for "tuned plate tuned grid oscillator".

After looking at 100s of oscillator circuits I've not been able to convince myself that the effect between a valve's anode and grid is the same as that between a transistor's collector an base.

"Therefore, if the plate is slightly more positive, the voltage induced on the grid (the opposite plate of this small "capacitor) will be negative."
http://electronicstheory.com/COURSES/ELECTRONICS/e101-58.htm

[Cyberdragon]

There is only one way to find out, datasheet time!

EDIT: Emitter-Base = 25pF
           Collector-Base = 8pF

[Mr J]

It's a Miller crystal oscillator.

Thats it!!!! thanks Cyber