Electronic – LC Oscillator Circuit Analysis


I've found this schematic of a LC sine wave producing circuit online, and I was trying to figure out how it works (particuarly the feedback cycle).enter image description here

Can someone tell me if this is right? I think I've figured it out to a point, but I'm not completly sure. Alot of the details on these circuits are quite fuzzy and dont really explain much. (When describing points on the schematic, I will use terms like "the top of the C1" to describe the part of the capacitor that appears closest to the top of your screen).

1: L1 begins to build a field, and as the current through L1 and Q2's gate rise, Q2 begins to open up and produces the beginning of a sine wave with a positive slope. At the same time, C1 is charging as well.

2: L1 begins to become saturated and the current remains constant, Q2 is almost completely open. The sine wave has just about reached its crest. C1 is near the end of charging and therefore Q1 begins to open up.

This is where I can't figure anything else out. How does the capacitor and inductor actually oscillate to produce this sine wave? It somewhat makes sense, but I cannot grasp the full concept when I try to fully analize the circuitry. Any help is appreciated. Thanks.

Best Answer

Q1 and Q2 are actually cross coupled because each transistor has its collector tied to the opposite transistor's base. This can be looked upon as a form of dynatron oscillator because R1 behaves like a constant current source. Where efficiency is more of a concern a choke is used for R1. If you tip the circuit upside down and redraw with PNP transistors then all will be clear to you.

There should be a cap across the inductor and the expected frequency is predictable. C1 is just a bypass cap and does not determine the osc frequency. Normally the output is taken from the junction of Q1 collector, Q2 base, and L1. Taking the output from where it's taken will give less output and lots of second harmonic distortion. This unorthodox output point may be useful if a frequency doubler is to follow.