What is the difference between CMOS logic IC 04 (inverter) and U04 (unbuffered inverter)?

CMOS logic ICs have an inverter as a standard function, and there are "04" and "U04". "04" is used for general logic processing (digital signal processing), and "U04" is mainly used for oscillators such as crystal oscillation circuits and RC oscillation circuits.

Fig. 1 Inverter "04" and "U04"
Fig. 1 Inverter "04" and "U04"
  • 04: It has a 3-stage inverter configuration, the first stage is optimized for the input, and the final stage has a function as a buffer.
  • U04 : Consists of a single inverter stage that has both an input stage and an output stage (buffer stage).

“04” and “U04” are configured as shown in Fig. 1. "04" has optimized MOS size for the required performance of input block and output. As a result, in the same group (family), the input capacitance of "04" is smaller than that of “U04”. "04" has a high gain due to its 3-stage configuration. In addition, the High-level / Low-level input voltage range (VIL, VIH) for judging "H" and "L" is wider than that of "U04" (Table 1), and the noise margin is high. However, the propagation delay time will be slightly longer.

Table-1 High-level/low-level input voltage (V<sub>IH</sub>/V<sub>IL</sub>04" and "U04"
Table-1 High-level/low-level input voltage (VIH/VIL) "04" and "U04"
Fig. 2 Conceptual diagram of the oscillation circuit
Fig. 2 Conceptual diagram of the oscillation circuit

Fig. 2 shows a conceptual diagram of an oscillator circuit that uses an inverter.

Disturbances such as noise at power-on become the seeds of oscillation and enter the input Vin. These seeds are so small that it is amplified in the linear region of the inverter, filtered in the feedback circuit and summed to the input. The conditions for oscillation are that the phase of the feedback signal is in phase with the input signal, and that the gain of the inverter is large (the magnitude of the "input" + "feedback signal" is greater than the original input signal).
“04” has a small input capacitance, so it has a wide band and high gain. It can oscillate up to a higher frequency than “U04”. Considering only this point, it seems that "04" is more suitable for oscillation than "U04". However, the linear region used for oscillation is outside the region shown in Table 1 (for example, at 4.5 V, "04" is 1.35 V to 3.15 V and "U04" is 0.90 V to 3.6 V), so "U04" is wider. The oscillation signal must circulate within this range for stable oscillation. Considering more stable oscillation at the desired frequency, it is recommended to use "U04" in the oscillation circuit.

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Please refer to the following e-Learning for inverter circuits and oscillation.
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Please refer to the following document for the basics of logic in general.
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