automatic compensation for impedance mismatches

[marios90]

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.

 

[Audioguru]

Your schematic is a nightmare of +10V and Gnd wires all over the place. Instead you should use Vcc and Gnd symbols.

 

[chuckey]

" that the voltage across both different resistors will be the same", Why not build two voltage regulators with a common reference voltage? The current will be what it is V/R, If the problem is that R can vary over a decade or so, so Vout must vary as well over a wide range, then use a single voltage reference which is derived or modified from the current in one of the resistors.
Frank

 

[FvM]

Apart from the incredible drawing style, the operation of the circuit remains mysterious to me.

OTA LM13700 has a maximum output current equal to the Iabc bias current (about 500 µA), so it can never keep up with the 4 mA source. On the other side, it has a maximum differential input voltage of +/- 5V, so it can be easily damaged by the differential amplifier output. I believe that some meters are measuring current and others voltage, but the schematic doesn't indicate it.

After noticing these details, I stopped decoding the circuit's operation principle.

P.S.: I forget to tell my conclusions. For the time being, I assume that the observed behaviour is expectable by (wrong) circuit design.

I think you should basically tell what you want to achieve and show a principle schematic of the intended circuit with input and output signals.