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Differential Instrumentation Amplifier with a Bridge Transducer

Figure 1.  Circuit Diagram for an Instrumentation Amplifier with a Bridge Transducer

The circuit shown in Figure 1 is a simple differential instrumentation amplifier that has a resistive transducer (Rt).  A resistive transducer is a device whose resistance changes when a certain physical energy applied to it changes. Common examples include transducers with resistances that vary with temperature, pressure, and light shining on it.

As in most bridge circuits, the components in this circuit's bridge network (consisting of Ra, Rb, Rc, and Rt) are chosen so that the bridge is balanced at a certain reference condition, i.e., Rc/Rb = Rt/Ra.  One way todothisistomakeRa=Rb=Rc=Rt=R at the chosen reference point. 

When the bridge above is balanced, Va = Vb, causing the input voltages to A3 to be equal and the output of A3 to be zero.  When the resistance of Rt changes, however, the bridge becomes unbalanced, causing a non-zero voltage Vab to appear across the inputs of A3. This, in turn, results in an output voltage Vo that is proportional to the change in resistance of Rt, i.e., Vo = (RF/R1)(?R/4R) Vdc where ?R is the change in Rt's resistance.

By attaching an indicating meter to the output of A3 and calibrating this accordingly, this circuit may be used to measure various physical quantities with the appropriate transducer.