Astable Oscillators

Trivially, the oscillators is use when you want to generate some periodic signal. Here the astable oscillators can generate the square wave depend on the RC negative feedback circuit. Let remind the schmitt trigger circuit, which we have positive and negative threshold. It will be the maximum charge and discharge for the RC circuit and cause the circuit to oscillate. The figure below show that the astable oscillator is the schmitt trigger with RC negative feedback.

Then the complete circuit is show below in which the periodic of the signal can be calculate with this formula.

Now we are going to analyse this circuit. First of all, you need to know the voltage across resistor in the RC circuit which is shown in equation 1. Then the output from schmitt trigger is L+ and L-. So the capacity try to charge and discharge to L+ and L- respectively. Unfortunately, it can charge to βL+ and βL- because of the schmitt trigger threshold. After that, it can create 2 equations in terms of charge and discharge in equation 2 and 3 respectively. From these, it can write in terms of charging (T1) and discharging time (T2) as equation 4 and 5. Finally, the period of the oscillate signal is T1 plus T2 will be equal to equation 6.

So, now we go to the design example here. The result is around 1 KHz. We use 2 Kohm as R1 and R2 and 4.7 Kohm as R and 100nF as C.

Finally, we do the experiment with this setup. Here is the result. The yellow line is the square wave which we want it to oscillate and the blue line is the charge and discharge voltage across the capacitor. Because β is equal 0.5, then the threshold value is 7.5 and -7.5 V (voltage rail is 15 V).

Thanks for reading.