How to use refrence values inside sed command

I am using script

S_db=`grep SRC_DB= ${login_txt} | cut -d= -f2`
T_db=`grep TGT_DB= ${login_txt} | cut -d= -f2`
today=`date +%Y%m%d`
sed 's/USER_WORK/RAR_WORK_D1/g' < ${my_path}/files/${T_target}_${today}_DDL.txt > ${my_path}/files/temp9

But when I am trying to remove hardcode values using script

S_db=`grep SRC_DB= ${login_txt} | cut -d= -f2`
T_db=`grep TGT_DB= ${login_txt} | cut -d= -f2`
today=`date +%Y%m%d`
sed 's/${S_db}/${T_db}/g' < ${my_path}/files/${T_target}_${today}_DDL.txt > ${my_path}/files/temp9

Its not working, Can u pleast tell why

Try with -e option...
use double quotes

sed -e "s/${S_db}/${T_db}/g" ..........

it should work