In fact, this is determined by the basic circuit structure. When a circuit operates at a lower level, it tends to have a lower loop impedance, which in turn means better noise immunity. To help you understand this more clearly, let's look at a practical example. Some of you might have learned a PCB layout rule that suggests, when possible, active-high lines should be kept as short as possible, while active-low lines should be extended. The reason behind this rule is that low-level signals typically have lower loop impedance and are therefore more resistant to interference.
For instance, consider an OC (open collector) or OD (open drain) circuit, where a switch controls the output level. With a pull-up resistor, the switch being on pulls the signal low, and when it’s off, the signal goes high. This ensures a stable, defined state, reducing the risk of the circuit becoming uncontrollable. In such cases, it's safer to have a high level rather than an undefined one. Structurally, OC circuits are advantageous because the collector is less likely to break down, making them more robust under stress.
In contrast, totem-pole output circuits can have both high and low levels with similar risks. However, some engineers still choose to add a pull-up resistor to create an OC or OD configuration. You might wonder why they don’t use pull-down resistors instead. That’s a great question—considering the different behaviors of high and low states, using a pull-up resistor can provide better control and stability in certain applications.
Another benefit of using a pull-up resistor in OC or OD circuits is the energy-saving effect. When the output is high, the transistor is off, resulting in almost no current flow. This makes the circuit more power-efficient, especially in low-power or battery-operated systems. So, even though the circuit may seem simpler without a pull-up resistor, the advantages of stability, safety, and efficiency often make it worth the extra component.
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