Marrying Gain and Balance

Figure 1 shows a basic resistor bridge. The circuit is usually credited to Charles Wheatstone, although S. H. Christie, who demonstrated it in 1833, almost certainly preceded him.1 If all resistor values are equal (or the two sides ratios are equal) the differential voltage is zero. The excitation voltage does not alter this, as it affects both sides equally. When the bridge is operating off null, the excitation’s magnitude sets output sensitivity. The bridge output is nonlinear for a single variable resistor. Similarly, two variable arms (e.g., RC and RB both variable) produce nonlinear output, although sensitivity doubles. Linear outputs are possible by complementary resistance swings in one or both sides of the bridge.