|
So last week I published a "worst working oscillator". @hurz commented that it works better at high frequencies, but probably wouldn't work IRL, and linked some examples. The examples were indeed much better, but were just the same circuit, just the component were different, so of course I wondered why. They had a 10pF capacitor, so I figured transistor internal capacitance must be making a difference.
Going back to my low-speed oscillator, I inserted capacitors from the base to the collector and emitter like the transistor has internally, but larger for the lower speed. It worked! The oscillator started working well at low speeds. It worked better still when I removed the collector-base capacitor, and then 5 minutes later I found it's not a new design: it's a Pierce oscillator.
I think the Pierce is normally drawn with the resonant components in a pi shape, but this way shows the feedback path more clearly, and the relationship to "worst working oscillator" too.
The capacitors don't have to be the same. Quick testing shows it's a little smoother if they are. The larger Cbg is relative to Ccg, the larger the signal.
|
