Astable Multivibrator/Square Wave Generator using Operational Amplifier

1. Astable multivibrator circuit using op-amp. It looks like a comparator with Hysteresis (Schmitt trigger), except that the input voltage is replaced by a capacitor. 2. The circuit has a time-dependent element such as resistance and capacitor to set the frequency of oscillation. 3. the comparator and positive feedback resistors R1, and R2 form an inverting Schmitt trigger. When V0 is at Vsat, the feedback voltage is called the upper threshold voltage VUT and is given as:- Vut=R1.(+Vsat)/R1+R2.............1. 4. When V0 is at - Vsat the feedback voltage is called the lower-threshold voltage VLT and is given as:- VLt=R1. (-Vsat)/R1+R2............2. 5. When power is turned ON, Vo automatically swings either to Vsat or to -Vsat since these are the only stable States allowed by the Schmitt trigger. 6. Assume it swings to Vsat with V0= Vsat, we have VP = VUT and the capacitor starts charging towards Vsat, through the feedback path provided by the resistor Rf to the inverting (-) input. This is illustrated in the circuit 7. As long as the capacitor voltage Vc is less than VUT, the output voltage remains at Vsat.