Propagation Delays

It takes a non-zero amount of time for a transistor to change state, due to the physics of transistor devices. The propagation delay is a measure of the time taken to change state:

t_PLH Time taken to go from L to H.
t_PHL Time taken to go from H to L.
The average propagation delay is

$\begin{displaymath}t_{P(ave)} = \frac{t_{PLH} + t_{PHL}}{2} . \end{displaymath}$

Obviously, the speed is limited by the propagation delay.

**Figure 140:** Propagation delay.
$\begin{figure} \begin{center} \epsfig{file=images/digdevimg2.eps}\end{center}\end{figure}$

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