MOSFET as a voltage controlled resistor

This question might be too localized, but I try.

Is it possible to replace a variable resistor by a MOSFET, under conditions shown in the following schematic?

If yes, can someone propose a MOSFET type or the required MOSFET parameters.

^{simulate this circuit – Schematic created using CircuitLab}

Update

What I am actually trying to accomplish is to replace R2a by something simple that I can control with a microcontroller (DAC).

I am hacking an existing device and can not replace the resistor R1.

Answer

Technically the MOSFET can operate as a variable resistor, but there are two main issues:

1. 
   

   In the ohmic region (which is quite narrow, in terms of output voltage) the linearity is poor, and it also depends on input voltage. It won't be very easy to tune it to behave like a proper resistor.

   
   


2. 
   
   

   MOSFETs' output resistance is usually not an accurate value, and it will be hard to get the exact value from the datasheet. What you can do is to measure it for various input and output voltages, and to create a table with the values. But if you don't need it to be accurate, you can use the graphs in the datasheet.

   
   

Another choice can be to use an integrated VCR.