the gain of any amplifying device, such as a vacuum tube, transistor, or op amp, can be increased by feeding some of the energy from its output back into its input in phase with the original input signal. this is called positive feedback or "regeneration". because of the large amplification possible with regeneration, regenerative receivers often use only a single amplifying element (tube or transistor). in a regenerative receiver the output of the tube or transistor is connected back to its own input through a tuned circuit (lc circuit). the tuned circuit allows positive feedback only at its resonant frequency. in regenerative receivers using only one active device, the same tuned circuit is coupled to the antenna and also serves to select the radio frequency to be received, usually by means of variable capacitance. in the regenerative circuit discussed here, the active device also functions as a detector; this circuit is also known as a "regenerative detector". a regeneration control is usually provided for adjusting the amount of feedback (the loop gain). it is desirable for the circuit design to provide regeneration control that can gradually increase feedback to the point of oscillation and that provides control of the oscillation from small to larger amplitude and back to no oscillation without jumps of amplitude or hysteresis in control.