I have a bottom gate TFT transistor: 

* W/l=320um
* Substrate: corning glass
* Gate: 60nm Molibdenum
* Dielectric: 350 nm Ta2O5 
* Semiconductor: 30nm IGZO 
* S-D: Molibdenum 60nm

It is wired like in current mirror setup https://en.wikipedia.org/wiki/Current_mirror#Basic_MOSFET_current_mirror : 

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![schematic](https://i.sstatic.net/34hBc.png)

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I am forcing pulses of current IDS and registering VGS=VDS
This is what it looks like on the SMU:

* Plot 1: CH1 IDS VS time
* Plot 2: Ch1 VGS vs time
 
[![Plots][2]][2]

I want to wire a second transistor in the same way, get a recording VGS2 and then find the difference VGS1-VGS2.

## The application
I am using the TFT to detect ionizing radiation: The X-ray radiation creates electron-hole pairs in the dielectric, which then affects the VGS recorded for a fixed ISD

Here is a plot showing how the VGS curve changes with the radiation exposure:

[![VGS vs Time][3]][3]


## EDIT-from remices2 answer ##
Does the circuit in the answer of remicles2 work? Since it has only gotten down votes for now, but I think because it wasn't using my original circuit (which I edited now since I think it was my circuit that was wrong.) 


  [1]: https://i.sstatic.net/Z0MZv.png
  [2]: https://i.sstatic.net/lpcKn.png
  [3]: https://i.sstatic.net/9GFJ5.png