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 to do this is to make Ra=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)(ΔR/4R) Vdc where ΔR 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.
See Also:
Instrumentation Amplifiers;
Operational Amplifiers
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