Hey guys I am making a project now in which I need to vary the voltage drop on the load. The schematics is simple, I attached it. The voltage V_L needs to vary between 0.26V and 3.6V. The current must be around 720nA. The circuit voltage is 5V, but can change if needed. I was thinking about using some FET as a voltage controlled resistor (changing the gate voltage changes the conductivity of the gate), but I can't find anything with Rds high enough. Following the requirmenets its Rds needs to ibe around 4.6MOhm and I find nothing even close to that. I thought I ask here, maybe one of you would know some series of FETs which I could check out. I would highly appreciate your help. Thank you in advance
edit :
I know that changing the value of the resistence of the FET would change the current. I am planning to use two FETs, the resistence of the first would be raised, of the second one reduced with the same value. This would keep the current constant.
Schematics and Requirments.pdf|attachment](upload://r3h6zu762cF9y31aenj8FaPRykM.pdf) (31.8 KB)
Hi, @arad2456
Please read the post at the start of any forum , entitled "How to use this Forum".
This will show you how to attach files,
It is better if your schematic is jpg format, then you can just drag the file into your post.
What is the application for such low current control.
I need to vary the voltage APPLIED to the load so you need to drop it across an active device.
You will not find a MOSFET with Rds of 4,600,000 Ohms.
Rds is the resistance of the MOSFET when it is turned fully ON, and the lower the better in the usual applications it is needed to work.
It is unclear (to me at least) what you are trying to achieve.
Are you wanting to vary the voltage (0.26V to 3.6V) with a low load current ~720nA. Or are you trying to get a constant current ~720nA with a load that will develop 0.26V to 3.6V?
I suspect it may be the latter. If that is the case then control with an op-amp may suit, this circuit could be suitable:
There are other constant current circuits uisng opamps as well. This is one of the most simplest.
How long has the current got to flow for? One technique is to use a constant ramping voltage (dv/dt)and a fixed capacitor (C).
I = C.dV/dt.
This only works while you mantain the voltage ramp. A Miller integrator is an easy and simple way to obtain constant voltage slope, other wise an integrator circuit.
A FET translates a gate voltage into a current, not a resistance. That's much closer to your needs than a controlled resistor. Unfortunately the relationship between the gate voltage and ds current depends on many parameters, not really useful in your project.
I am making a project now in which I need to vary the voltage drop on the load. The schematics is simple, I attached it. The voltage V_L needs to vary between 0.26V and 3.6V. The current must be around 720nA. The circuit voltage is 5V, but can change if needed. I was thinking about using some FET as a voltage controlled resistor (changing the gate voltage changes the conductivity of the gate), but I can't find anything with Rds high enough. Following the requirmenets its Rds needs to ibe around 4.6MOhm and I find nothing even close to that. I thought I ask here, maybe one of you would know some series of FETs which I could check out. I would highly appreciate your help. Thank you in advance 
