automatic compensation for impedance mismatches

Hi,
i need urgent help if you can help me. I am working on a project for which i am trying to built a circuit that will automatically compensate for impedance mismatch between two resistors by varying the current provided to each resistor so that the voltage across both different resistors will be the same. I have attached the circuit design. The total current in the circuit should not exceed 8mA. However, in building the circuit a get negative voltages across resistors R1 and R2 which mess up everything and i cannot identify the problem. Can anyone help me in pointing the problem?? I NEED HELP PLEASE!
The idea is that i have two fixed current sources providing 4mA current each and then a fully differential amplifier with a gain of 10 has as output the difference in voltage between R1 and R2 which is then fed to a transconductance amplifier for either injecting or drawing the appropriate current so that the voltage across R1 and R2 would be the same for a resistance tolerance of +-20%. However, i cannot understand why i get negative voltage across the resistors in practise as i have tried in multisim and it seems to work in simulation.

The simplest way to have the same voltage across two resistors of different value is to just put them in parallel with a constant voltage source.

But I'll assume that your needs don't permit that. But what are your needs and limitations?

As for the differences between a simulation and real hardware, you have to understand the the impact that real world parasitics have on circuit behavior that may not be modeled in a simulation. For instance, I see you have two 4mA current sources on your schematic. How are you implementing them in hardware? In simulation a current source will probably be accurate down to zero volts, yet a hardware current source will need some minimum voltage headroom to work.

Hi, thanks for your reply. My need is that by varying the resisor value of one of the resistors the circuit will automatically compensate for the resistor mismatch by varying the current provided to each resistor so that the voltage across them will be the same. My limitation is that the total current should not exceed 8mA. I am implementing the fixed current sources in practise using lm334 current source. I also have the idea to use XTR111 voltage to cirrent converter by feeding it with a fixed voltage to provide a constant current and act as a fixed current source