Linearization in analog circuits is difficult unless there is a relatively simple
equation to describe the sensor’s characteristics. In some applications a much
more expensive linear transducer may have to be used due to the inability of
analog circuits to linearize the signal conversion. Figure 12.3a shows the circuit
of a logarithmic amplifier. Figure 12.3b shows the variations in characteristics
with various resistor values that can be obtained for use in signal
linearization. When R2 = ∞ and R3 = 0, the amplifier has a logarithmic relation
between input and output. When R3 is larger than zero the gain is higher at the
upper end of the scale, as shown. If R2 is a high value, the effect is to reduce the
gain at the lower end of the scale. Multiple feedback paths can be used with nonlinear
elements and resistors to approximate the amplifier characteristics to
those of the sensor.