14 To convert an amplifier into an oscillator, you must connect a portion of the output signal to the input. This feedback signal must be in phase with the input signal to induce oscillations.
Figure 9.11 shows three methods you can use to provide a feedback signal from a parallel LC circuit. Each is named for its inventor.
In the Colpitts method, the feedback signal is taken from a connection between two capacitors that form a voltage divider. In the Hartley method, the feedback signal is taken from a tap partway down the coil, or from a connection between two inductors. Therefore, an inductive voltage divider determines the feedback voltage. The Armstrong method uses a step-down transformer (an inductor with an extra coil with fewer turns than the main coil). In all three of these methods, between one-tenth and one-half of the output must be used as feedback.
15 The output voltage appears at one end of the parallel LC circuit shown in Figure 9.12, and the other end is effectively at ground. The feedback voltage Vf is taken between the junction of the two capacitors. ...