Tempdb files Solid state on Same drive?
There are several factors that comes to play here:

How much I/O in general can a device (your SSD in this case) handle? Say that out of 100% in total for tempdb, the log (ldf) caters for 30%. Buying a new SSD and put the ldf on there will give you more IOPS and those 30% for the ldf won't be taken from the drive where the mdf/ndf is.

The "separate the log" made a lot of sense when we had single disks and there was a huge performance difference between sequential and random I/O. A transaction has to wait for the ldf records to be written to the disks (hardened). The ldf is (almost) only sequential I/O. Isolating ldf to a separate disk means the disk is in position for the next write operations meaning sequential I/O (no moving of the disk arm). How big difference is there between sequential and random I/O for an SSD disk? I don't know, but probably minuscule. So, this aspect is probably not relevant for modern disk subsystems.

So, IMO, it boils down to capacity. Can your SSD handle the I/O load with reasonable response time? If not, then you want more capacity - more SSD drives.