Single tube LC self-excited oscillator circuit! R1,R2,R3 constitute the static operating point of BG1, L,C1 is the resonant loop, C2 is the positive feedback capacitor, and C3 is the signal output.
When the power is switched on, the instantaneous LC loop will generate a attenuated oscillation current signal of charge and discharge. The signal will form a feedback on R2 and be sent to the input end of BG1 through C2, and the signal will be amplified and sent back to the LC loop to make up for the attenuated signal. This oscillation can be maintained continuously, which is the principle of self-excited oscillation. Among them, the size of C2 is very important, too small to vibrate, too large circuit blocking.