Comparator / Operational Amplifier - RC Astable Multivibrator (1)

This is an RC relaxation oscillator using a comparator or an operational amplifier, the circuit is configurable for single or dual supply operation. The difference between the periods in single and dual supply modes is fairly small. Alternatively, the top terminal of Rrefp can be strapped to the positive power supply terminal of the comparator/operational amplifier and the bottom terminal of Rrefn can be strapped to the negative power supply terminal of the comparator/operational amplifier. Switch S2 can be eliminated when using this wiring. Operation at low frequencies may necessitate the use of polarized electrolytic capacitor. In this case, when the circuit is using a dual-polarity power supply, the negative electrode of the capacitor should be connected to the negative supply terminal. Switch S3 selects between nonpolarized and polarized capacitor. The negative electrode of the polarized capacitor is not committed to ground, instead, it is strapped to the negative supply terminal of the comparator/operational amplifier. This way, the negative electrode of the polarized capacitor is properly connected regardless of the position of switch S1. When using a comparator, the circuit shown is for push-pull output drivers. For open-collector/drain output comparators a pull-up resistor from the output to the positive supply is needed. The pull-up resistor provides the charging path for the capacitor. If differences in timing relative to the circuit shown is to be minimized, the pull-up resistor should be as low as possible (subject to current sinking capability of the operational amplifier and power consumption consideration).