Amplified
Amplify, ample, and amplification all have the same root word as amplitude, which is "ample".
Amplified
frequent, frequenting, frequently
Three words that have the same root word as "trough" are "troughed," "troughs," and "troughing."
frequent, frequenting, frequently
erupt, abrupt, corrupt
Some words that have the same root words as poverty include: Impoverished Impoverishment Impoverish
I have seen the word "doublet" used to mean two words that come from the same root word.
They're not. The same amplitude can have high or low frequency, and the same frequency can have large or small amplitude.
The waves with the same amplitude are the ones that have equal height or intensity.
electricity, electric
umm, what words? here's a couple for you: induction, conduction, deduction, ..