Waveform Generators

Op-Amp Astable Multivibrator

This multivibrator provides a square-wave signal with an output voltage swing of approximately 0 to V_CC. The multivibrator circuitry consists of an op-amp, IC₁, and the associated components shown in the figure above. Since a single power supply (V_CC) is used for the op-amp, equal value resistors, R₁ and R₂, are used to provide a bias voltage at the positive input of the op-amp equal to one-half of the supply voltage.

The multivibrator circuitry waveforms are presented in the figure above. Resistor R₃ provides positive feedback and, in conjunction with resistors R₁ and R₂, determines the swing of the square-wave voltage about the bias point at the positive input of the op-amp. When R₁, R₂, and R₃ are equal, the swing of the voltage at the positive input is from V_CC/3 to V_CC*2/3. The charging and discharging of the R-C network, R₄ and C₁, produces an almost triangular waveform at the negative input of the op-amp. A change of state of the op-amp output occurs when the negative input voltage level crosses the voltage at the positive input of the op-amp. Thus, the op-amp functions as a comparator, operating in a saturated switching mode, at a frequency f determined by the R₄C₁ time constant. When R₁, R₂, and R₃ are equal, the frequency can be calculated as