How to use sed to append a line in bash script with a variable

Question

i am just trying to append a line from my bash script:

sed -i '$ a  TEXT'  "filename"

but when TEXT is a variable it does not work

sed -i '$ a  $VARIABLE'  "filename"

i have tested endless iterations with ',''," before/after $ and ${}, escape with \ etc., but i always get a literal $VARIABLE in my file

ps. please do not propose to use echo:)