# How will supply voltage affect propagation delay?

Here is my circuit:

I want to measure the propagation delay between the input D and output Y.

As I understand it, the higher the supply voltage VDD is, the faster the propagation is.

When I measure the propagation at different supply voltages, that is not the case.

My plot of propagation delay time between the input D and output Y at different supply voltages VDD looks like this:

• More voltage means, more current, means faster dV/dt for all gate capacitances. Nov 16, 2021 at 12:42
• Your double inverter in the middle is not active. You have used the node "BC_bar" before and after it, forming an invisible short-circuit. Nov 16, 2021 at 12:49
• Does this answer your question? Why does decreasing the CMOS supply voltage also decrease the maximum circuit frequency? Nov 16, 2021 at 13:24

The graph shows exactly what you describe.

Higher voltage, shorter propagation delay.

• Below about 1.6V, no signal goes through at all. The graph shows zero nanoseconds of propagation delay, but that isn't real. It's just how the simulation represents the lack of output.
• At just above 1.6V, the circuit starts working and has a (relatively) long propagation delay of 7 nanoseconds.
• As the voltage goes up, the time of the propagation delay gets shorter - the signal gets through the circuit faster. It is down to less than 1.4 nanoseconds when the voltage is at 3.15V.
• Sorry what do you mean by no signals goes through at all, does it mean that the whole circuit didn't turn on when supply voltage is below around 1.65v? What I am confused about is that before 1.65 volts why the propagation delay is increased when increasing the supply voltage Nov 16, 2021 at 12:52
• Below about 1.65V, the circuit didn't work at all.
– JRE
Nov 16, 2021 at 13:10
• I see,thank you !! So, just a quick question, will the width and length affect the propagation delay or switching time as well? Nov 17, 2021 at 11:47
• I know enough about MOSFETs to know that "width" and "length" refer to feature sizes - and that's it. You'll have to ask someone else about how MOSFET geometry affects speed and propagation delay.
– JRE
Nov 17, 2021 at 13:17