This circuit (shown on Picture 1) is an oscillator that generates a square wave. It is also known as an astable multivibrator. The op amp works as a comparator. Let’s assume that the op amp output goes high on power on, thus making Vo = +Vz, where Vz is the breakdown voltage of the zener diode. The capacitor charges with a time constant τ = R4C1. When the capacitor voltage reaches βVo, where β = R2/(R1 + R2), the op amp output switches low, and Vo = −Vz, as shown in the graph on Picture 2.
Picture 1: Square Wave Generator circuit
The zener diodes used in this circuit, as shown on Picture 1, are of type EDZV6_2B with breakdown voltage of 6.2 V. The time constant with which the capacitor C1 charges is equal to τ = R4C1 = 1 ms. The value of beta factor for this circuit is β = R2/(R1 + R2) = 1/2, so, the capacitor voltage should reach half of the output voltage.
Picture 2: The output and capacitor voltages
Time-domain analysis
The results of the transient analysis for this circuit in time domain, are shown on Picture 3. The output voltage starts to increase at about 33 ms. The first peak of the output voltage is about 4.51 V and it's reached at time of about 39.3 ms. After about 42 ms the output voltage continues to oscillate with period of about 2.65 ms, which means that the frequency of the oscillations is about 377 Hz. The maximum and minimum values of the Vo are about +/- 5.4 V. The voltage of the capacitor C1 oscillates with same frequency, with values between +/- 2.7 V. The voltage of the zener diode D2 varies from about + 4.9 V to about -500 mV, when the output voltage reaches it's negative peak.
Picture 3: Transient analysis - output voltage wave form (time-domain)
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