Hello. I have a question about an astable oscillator made with a 555 timer IC. According to a National Instruments website, the off-duty cycle for an astable oscillator is Rb / (Ra + 2Rb) and the frequency is 1/(0.7*(Ra + 2Rb)*C), where Ra is the resistor in between pin 7 and the power source, and Rb is the resistor in between pins 7 and 2/6, and C is the capacitor in between pins 2/6 and ground. I believe this is a standard setup for an astable oscillator. However, I've noticed that oftentimes Rb >> Ra, so my question is, is there a reason for this? Because if Rb >> Ra and you calculate the duty cycle and frequency of your circuit, the influence of Ra in these cases is often negligible.