Amplifer help!

[sid2286]

I'm working with a circuit, it kinda works ok...my a detailed look at the output doesnt sounds ok.

I've a bridge input to a differential amplifier ,R6 and R7 are used to balance the bridge along with R8 and R9.

Now when i unbalance the bridge using VR1, there is a change at pin No. 7.

but the change is non linear, initially there is a very little change and as i go on increasing the pot, the change increases.

I want to have a linear change, all the resistor are of 10K except 220E at R8 and R9.

How do i do that??

 

[duke37]

You could get rid of R8, R9 and R10 and connect the pot directly to the op-amp.
Putting R10 between the pot and the junction of R8 and R9 would reduce the sensitivity of the pot setting.

 

[sid2286]

well...I removed the resistors and it works gr8!

But the real problem is I need to have R8 and R9 resistor in circuit, as they are used for balancing a Wheatstone bridge circuit.

whatelse can i do?

Regds,
Sid

 

[duke37]

This part of the circuit just supplies a voltage as a proportion on the input voltage. If you have a bridge, you should be measuring the voltage across the bridge.

What else? Show us the bridge.

 

[sid2286]

Thanks for your comment!

I have attached the bridge circuit, the sensor filament act as two resistor and R8 and R9 are used to balance them.

once there is a chemical reaction on the filament, the bridge is unbalanced and thats how the output is taken.

Please suggest, how I should go about with it?

Rgds,
Sid

 

[duke37]

The upper and lower resistors in the detector are presumably similar so the output will sit about half the supply voltage. This is compared to the adjustable circuit.

You have divided down the detector output with R6 and R7 so things are unlikely to match. Why not take the detector output directly to the amp?

R10 goes to the + input of an amp and will be passing negligible current so will drop negligible voltage. Get rid of it.

If you do another scan, please put it the right way up - I have a stiff neck !!!!!!!!!!!!

I would say you are very close to getting it working.