The transistor generator and active resistance

In a real circuit free oscillations die away because of active resistance; to keep them going the lost energy must be returned from a source at the right moments. In a circuit with a frequency of hundreds of thousands of hertz, switching a key hundreds of thousands of times per second is impossible for mechanical devices; an electronic switch, the transistor, does it. In the generator the L–C circuit is connected to the battery through the transistor; the changing current in the circuit induces an EMF in the coupling coil Lb, which is applied to the transistor base and opens and closes it at the right moments. So a process at the output (in the circuit) controls the input: this is called feedback, and for the oscillations not to die out the input and output voltages must differ in phase by 180°. The frequency is still given by Thomson's formula. Such a system is an auto-oscillating system: it needs no external periodic action, the energy source is part of the system and the system itself regulates how the energy is supplied; its parts are an energy source, an oscillating system (the circuit) and an electronic switch (the transistor).