The pull-up transistor of the totem-pole output is used-to turn on the LED and the pull-down transistor is used to turn Â·off the LED. The lower impedance and higher current handling capability of the saturated pull-down transistor is used as an effective method of transferring the charge from the LED`s anode to ground as its dynamic resistance increases during turn-off. The slightly higher output impedance of the pull-up stage ensures that the LED is not over peaked during the less difficult turn-on transition.
This asymmetric current handling capability of the output stage with its variable impedance substantially reduces the pulse-width distortion and long-tai)ed response. As the signal propagates through two NAND gates, each transition passes through the high-to-low and low-to-high transition once, normalizing the total propagation delay through the circuit.