A very simple tone generator which uses two RC networks connected in series to obtain the necessary phase shift can be designed using this circuit diagram. Frequency range of the tone generator is between 20 Hz and 20 kHz, and distortion is kept to a minimum by using additional stability in amplitude.
Operational amplifiers A1 and A2 are the basics for the two phase shift networks. Double potentiometer P1 sets so that each network frequency signal phase offsets of 90 °. Select the desired switch S1: 20 Hz ... 200 Hz, 200 Hz ... 2 kHz or 2 kHz ... 20 kHz. Operational amplifier A3 provides an additional phase shift 180 ° and amplify the signal so that the system will maintain oscillation. To suppress HF oscillations above 100 kHz, the feedback loop of A3 is introduced capacitor C5.
Outputs of A2 and A3 is rectified by diodes D1 and D2 before reaching through P3 to the inverting input of A4 that compares feedback signal with a fixed reference voltage of zener diode D4.
Output of the operational amplifier A4 pass transistor T1 in conduction commanding amplification of A3 and maintains its output at a constant level.