The figure shows a square wave generation circuit. The LM139 series can be used for oscillation circuits of several MHz. This circuit is a square wave generating circuit that uses a few components. The output frequency of the circuit is determined by the time constant of R4, C1, and the hysteresis circuit determined by R1, R2, and R3. The maximum operating frequency is limited by the large signal propagation delay of the comparator. In addition, the operating frequency is also related to the nature of the load. In the case of capacitive loads, the maximum operating frequency will be reduced.
In the circuit, Figure (a) shows a single-supply circuit and Figure (b) shows a dual-supply circuit. The operating principle is the same, but the reference of the output waveform is different. In Figure (a), the lowest level is 0 and the highest level is Vcc. In Figure (b), the lowest level is -Vcc and the highest level is Vcc. When the selection of components in the circuit satisfies: R1 = R2 = R3 and R3, R4> 10R5, the oscillation frequency is:
Fo=1/2&TImes;0.694R4C1
The corresponding fo for R4 and C1 are different values ​​as shown in the table.
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