Which has least propagation delay?

least propagation delay

Which has least propagation delay?

a)ECL b)TTL c)RTL d)CMOS

Here are the propagation delays for these gates
a) ECL = 2ns

b) TTL = 1.5-33ns depending on the type of TTL. Conventional TTL is 9ns, Advanced Schottky TTL is 1.5ns

c) RTL = 25ns

d) CMOS = 5-20ns depending on if it is conventional CMOS, TTL pin compatible CMOS, high speed TPC CMOS or TTL compatible CMOS

So the fastest would be the Advanced Schottky TTL (74ASxx) at 1.5ns but the choice simply said TTL which I would interpret as Conventional TTL (74xx/54xx) which would have a propagation delay at 9ns.

So the winner is ... (a) ECL which has a propagation delay at 2ns. It is an old technology and the speed is due to the fact that the transistors never enter saturation -- so there is no time to wait for the transistor to come out of saturation. However, this logic works with negative power supplies and does dissipate more power than other logic families.

Best regards,
v_c

ECL is the fastest logic among all as you mentioned in list... since transistor is not allowed to saturate...if aollwed to saturate it will take its time to come out of that saturate region....