no it is not appropriate. I give any extra amount in between birthdays, I put the amount into their joint account. Birthdays are small personal gifts, like a book or a l`occitane set. Also giving large gifts on birthdays could lead to subtle expectation, particularly if there has been a pattern.
I have given large cash gifts, dates need to be noted for the 7 year rule btw. I also send a little note ie to use the cash wisely. There was no definite timing for these gifts but it is obvious that I have stopped but birthdays carry on with loving personal gifts. They are very grateful, I am very happy to have helped