Phase shift and amplification gives oscillations. Common emitter amplifier and a Phase shifting resistor -capacitor network shifts the phase first 180 degrees more rc networks shifts it again before feedback to the transistor closes the loop. Potentiometer gives some adjustment of frequency. Integrated logic inverter IC packages like 7404 or 7414 is an easy way to implement phase shift oscillators. Unity gain high input impedance output buffer with an opamp added as output connection.
