A keeper transistor, often used in digital circuits to maintain the stability of a node’s state, increases the HL (high-to-low) propagation delay due to its role in sustaining the voltage level. Here’s a concise explanation:
1. Function of the Keeper Transistor:
The keeper transistor holds the node voltage high when there’s no active drive, preventing unintended state changes caused by noise or leakage currents.
2. Impact on HL Transition:
During a high-to-low transition, the pull-down network must overpower the keeper transistor to discharge the node to a low voltage. This creates additional resistance, requiring more time to overcome the keeper’s hold on the high state.
3. Resulting Delay:
The added effort to discharge the node due to the keeper transistor’s presence increases the time it takes for the output to transition from high to low, thereby increasing the HL propagation delay.
Conclusion:
The keeper transistor’s design ensures signal integrity but at the cost of increased HL propagation delay. This trade-off is typically acceptable in applications where noise immunity is critical. For performance-critical circuits, designers must carefully balance the benefits of a keeper against the impact on delay.
