The Hartley circuit

A method of obtaining controlled feedback at RF is shown in Fig. 1. At A, an NPN bipolar transistor is used; at B, an N-channel JFET is employed. The PNP and P-channel circuits are identical, but the power supply is negative instead of positive.

Fig. 1: Hartley oscillators. At A, NPN bipolar transistor; at B, N-channel JFET.

The circuit uses a single coil with a tap on the windings to provide the feedback. A variable capacitor in parallel with the coil determines the oscillating frequency, and allows for frequency adjustment. This circuit is called a Hartley oscillator.
The Hartley oscillator uses about one-quarter of its amplifier power to produce feedback. (Remember, all oscillators are really specialized amplifiers.) The other three-quarters of the power can be used as output. Oscillators do not, in general, produce more than a fraction of a watt of power. If more power is needed, the signal can be boosted by one or more stages of amplification. It’s important to use only the minimum amount of feedback necessary to get oscillation. The amount of feedback is controlled by the position of the coil tap.