How do I bridge two different amps together?

Wheatstone bridge comparator?

• ok, so a wheatstone bridge is set up as two voltage dividers consisting of two resistors each, with each divider parallel to the other. when the bridge is balanced the voltage between the two center points of the dividers should be zero. so when i connect a 741 op amps input terminals, one in the center of each divider network, the difference between the two input terminals should be 0V. now, if an op amp is supposed to amplify the difference between its inputs then why does a balanced bridge drive it into high saturation? and other bridge conditions (or so multisim seems to tell me, along with me schoolbooks) drive it into low saturation?? it seems to be doing the opposite of what i understand an op amp to do. what am i missing??

• Answer:

  Without seeing your circuit I can't be sure that I am answering your question, but here goes. There are two issues. The first is the gain of the op-amp. An op-amp like a 741 has a very high gain. It could easily be 1,000,000 or more. Let us suppose your bridge is balanced to within 1 mV. That difference is amplified by 1,000,000 to give 1000 Volts. Obviously the op-amp just goes into saturation. The other problem is the input offset voltage of the op-amp. Due to manufacturing variations the two inputs need to be at slightly different voltages to get the output to its mid position. This means that even if you connect the two inputs together the output will be in saturation (could be + or -). Assuming the bridge resistance is fairly low connect the two nodes of the bridge to the two inputs of the op-amp through two 100k resistors and connect a 1M resistor from the output of the op-amp to its inverting input. The output voltage will then be minus ten times the "out of balance voltage" of the bridge.