Pronounciation questions, please help?

Do diD
Just wondering, do these two "D"s have the same pronounciation?
They seem different to me, cos the second one doesn't vibrate the voicbox, whereas the first one you need to vibrate your voicebox. Am I correct?

The second "D" sounds like a "T" to me. Can someone please clarify this, most preferably if you can give me the IPA for these two words. In addition, is the "D" sound apsired all the time in a sentence? For example:

I woulD like to help u.
I diD not .....
etc.

Thank you very much!