I'm hoping someone can help me remedy this problem (or at least come up with a temporary solution), as I am a novice in electronic circuits.

The below circuit shows my setup. **Vin** is my input signal which ranges from 1-5VDC, and it goes through a voltage buffer and then a 250Ω which converts the signal into a 4-20mA output **Vout**. 

![schematic](https://i.stack.imgur.com/Pjb5F.png)

[simulate this circuit](/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2fPjb5F.png) – Schematic created using [CircuitLab](https://www.circuitlab.com/)

This circuit output was working as expected for most of the range for 1 to <5V, but as soon as the signal starts to reach 5V, the voltage stays stuck at ~4.79V (~19mA) and never gets to 5V (20mA). From this, I learned that Op amps have an output impedance which I assume causes this voltage drop problem. I calculated an output impedance of about ~11Ω in the TSV612A which -added with the 250Ω- causes the voltage to max out at 4.79V for an input signal of 5V.

One solution that worked was to increase **Vop** (Supply voltage of the Op amp) to 5.5V, so that it had a little more voltage to work with in order to compensate this drop.

My question is this: is there another solution that will help to fix this voltage issue without increasing the 5V supply on the Op amp or changing the 250Ω resistor? Are there any characteristics of an Op amp that I could be forgetting about that might help me remedy this problem?

Thank you!