Ring oscillator

A ring oscillator is a circuit composed of a cascaded chain of inverters (logical NOT gates) arranged in a ring, such that the output of the inverter at the end of the chain is fed back into the first inverter, which produces an output at the output of each inverter that oscillates between two voltage levels representing true and false.

If the inverters used are buffered, then any odd number of inverters can be used. However, if the inverters used are unbuffered, then an odd number of at least 3 inverters must be used. (For simplicity, this article may simply say an "odd number" and ignore this caveat.) This is because a single unbuffered inverter in a loop with itself will simply have its output voltage equal its input voltage. Another formal proof of why a single unbuffered inverter won't work: the phase of the transfer function of a single unbuffered inverter doesn't cross 0° (or any integer multiple of 360°), so it fails to satisfy that necessary condition of Barkhausen stability.

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