Page 435 - Op Amps Design, Applications, and Troubleshooting

Antilogarithmic Amplifiers 411

FIGURE 11.4 Oscilloscope display showing the actual behavior of the circuit shown in Figure
11.3. (Test equipment courtesy of Hewlett-Packard Company.)

age. If the transistors are similar (e.g., a matched pair integrated on the same die),
then the effects of saturation current (/£$) will be eliminated. This is particularly
important because this parameter varies directly with temperature.
There are numerous applications for logarithmic amplifiers. One of the most
common is for signal compression. By passing an analog signal through a logarith-
mic amplifier, a very wide range of input signals can be accommodated without
saturating the output For example, an input swing of 1.0 millivolt to 10 volts might
produce a corresponding output swing of 0 to 8 volts. While this may not sound
impressive at first, realize that the smaller signals will receive much greater gain
than the larger signals. In this particular example, the relationship is 2 volts per
decade. Thus, a 1-millivolt to 10-millivolt change on the input will cause a 0-volt~2-
volt change in the output. Similarly, a 0.1-volt-l.O-volt input change will cause a 2-
volt (4 V to 6 V) change in the output. Since the smaller signals are amplified more,
the signal-to-noise ratio can be improved. More specifically, the smaller signals
become larger relative to the noise signals. When the composite signal is subse-
quently translated to its original form, the noise will also be reduced.
Another application for the logarithmic amplifier is to convert a linear trans-
ducer into a logarithmic response. Optical density of microfilm, for example, is
measured as the logarithm of the light that passes through the microfilm. By using
a light source to shine through the microfilm and onto a photodiode whose
response is linear, we have an output voltage (or current) that varies linearly with
optical density. Once this waveform is passed through a logarithmic amplifier, we
will have the required logarithmic relationship that represents optical density.

11.4 ANTILOGARITHMIC AMPLIFIERS

An antilogarithmic amplifier provides an output that is exponentially related to
the input voltage. If, for example, a linear ramp were passed through a logarith-