I'm looking for help designing a constant current pulse through a variable load impedance of 10-100k (R4). The ideal response is shown below. This waveform was measured from an off the shelf instrument without access to the design files.
The attractive component is the square wave that it generates. It spends a solid couple of milliseconds at my intended current.
My current design results in a circuit response as shown below with a sharp rise then fall (not ideal).
I'm hoping for my circuit response to more closely match that of the response shown above. What modifications could be made in order to square out the waveform?