Your Voltage to Current (Q1) has many problems that you are probably tired of hearing about.
The gain of the transistor is a major player in the temperature problems. Also the B-E voltage is a problem.
By adding a op-amp and resistor; the Vin is across R1. So the current is very predictable. Q1 gain is not in the formula. Also the B-E voltage is of no importance. Vin can not go very large. If you need 0 to 15V add a voltage divider. (two resistors)
Sorry not so simple but very linear and stable. Op-amp must work with inputs near ground. So chose the right part.
There is a two transistor (no op-amp) version if you are interested.
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