The output voltage produced by the original oscillator is not exactly a rectangular wave. In any case, a CMOS gate does not have this problem. This forces designers to use bigger capacitors to achieve lower frequencies. Having lower input impedances creates a more noticeable load effect on the RC circuit, and a more appropriate model for it would have to account for that lower input impedance.Īn implication of using TTL ICs for an RC oscillator is that the value of R is limited to smaller values, typically under $$2k\Omega$$. The problem with this parameter (input impedance) is that it has an impact on how accurate the series model is for the RC charge/discharge circuit. ![]() This results in input currents always lower than 100nA. These gates can have an input current as high as 1mA and as low as 0.1mA.ĬMOS gates, on the other hand, are made with MOSFETs, which have an extremely high input impedance-always higher than $$10M\Omega$$. TTL gates are made with BJTs, and despite the variant used in the LS family is aimed for low power, their input impedance is not very good. TTL vs CMOSĪnother reason for using the 4093B IC instead of the 74LS14 is their implementation technology. Note that the idle state in the output is high. In Figure 4, the extra input is used as an enable line. RC oscillator with the 4093B CMOS quad NAND integrated circuit. Whenever this enable line is high, the circuit will output the clock signal, and when it is not, a fixed high value.įigure 4. This gives the circuit the feature of an enable input line. $$T=RC \ ln \left ( $$ is 1.65V.īecause the 4093B is a two-input NAND, it can act as the original inverter when the other input is high. In the previous article, Exactly How Schmitt Trigger Oscillators Work, we went over the schematic of a basic RC Schmitt trigger oscillator and derived a general equation for the period and frequency. ![]() These oscillators are especially important because they are present in the internal oscillator in many popular MCUs. This article discusses the good and the bad regarding Schmitt Trigger RC oscillators.
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