# RC time constant and delay

I understand that for an RC circuit acting as a load of a driver will afftect the delay of the output after the RC load.

So as long as RC for different values of R,C are equal, delays are equal, is this right? For example R = 8 Ohm, C = 0.25p, RC = 2 and R = 5 Ohm, C =0.4p, RC=2

I simulate with these above 2 configurations of RC values but i get different delay values after the RC load, and also different rise/fall time.

I think that delays in these 2 cases should be equal and so do rise/fall times.

Are they any constraints on the R, C for to compare delays based on different RC time constant.

• $\tau = RC = 8 \times 0.25p = 2~ps$ (pico-seconds, not seconds). – Transistor Aug 3 '16 at 10:45
• An very very ugly circuit diagram - My eyes are hurting. – JIm Dearden Aug 3 '16 at 10:50
• You can click on the image to enlarge it – vqtuyen Aug 3 '16 at 10:52