# Is it possible to build a inverter that operates at a frequency set by an LC tank using ONLY that LC tank and transistors?

I was always dissatisfied with inverters because it seems like an LC tank and transistors would be all you need. However, after much time spent simulating and a few false positives experimenting, I am beginning to think it is flat out impossible. I have no way to prove it, but it seems that you need at least 4 passive components to create AC from DC. It seems as though it is often that there are a pair of passive components for each phase of the duty cycle.

Is this indeed the case?

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It sounds like you are talking about the extra components that provide a positive feedback path to keep the oscillator going. –  mng Apr 4 '12 at 19:10
I am not sure. What you mean. Can you give an example? –  Feynman Apr 4 '12 at 19:27

A blocking oscillator is able to generate a 'dirty' sine wave with only one transistor, a capacitor and an inductor. However this isn't an inverter - it produces a free running signal, but that signal is tuned by the LC time constant.

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That is interesting. Not exactly an isolated LC tank, but if that is the closest thing out there then thanks for showing me. –  Feynman Apr 5 '12 at 10:15

A LC tank circuit is just a inductor and capacitor connected to resonate at a particular frequency. These are not active components. They can not make a frequency that doesn't already exist. They can filter, sometimes with quite high Q, a mix of frequencies to pick out or reject a narrow range.

In other words, A L-C tank can be used as the governing elements that set the frequency, and also to filter out spurious frequencies to leave you largely with a sine, but the original signal with the desired frequency content can only be made with something that consumes power. This requires at least one active device somewhere.

If it were a desing criterion for some reason, it should be possible to create a circuit that oscillates and drives a L-C tank with a single active element. It will be easier with multiple active elements or multiple windings around the same core. If you are looking to have it produce significant power, I wouldn't worry about the number of active elements and think about drive level and efficiency instead.

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The goal here was to limit the number of passive elements to a single LC tank--not the number of active elements. I know I need active elements, and I am fine using as many as I need. I was however interested to know whether one could design an inverter using an LC tank as the only passive element. –  Feynman Apr 4 '12 at 22:34