Miniaturized logarithmic count-rate circuit

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Abstract
A four-decade, logarithmic count-rate circuit with a log transistor as the feedback element in an electrometer amplifier was designed and constructed for measuring periodic or random inputs. This new design enables a much greater degree of miniaturization than possible with the conventional, parallel diode-pump configuration. Pulses of constant charge are supplied to the log electrometer by a common-base transistor current source that is driven by a stable univibrator. Frequency compensation of the electrometer is not required because of the stabilizing influence of a 0.1 μF feedback capacitor that maintains the output voltage ripple at < 1 mV peak-to-peak for periodic inputs. The electrometer output changes 1 V per decade from + 4 to 0 V as the average input rate increases from 10 to 105 counts/sec. From 0 to 50 °C the total output voltage zero drift is < ± 10 mV and the 1 V per decade calibration changes < ± 10 mV. The maximum error of the logarithmic conversion is < 1% referred to the output voltage. A figure of merit for the output response to a step change in input frequency is defined as the number of pulses required for the output to reach 90% of the difference between the initial and final output voltages. This response number is plotted as a function of the ratio of the initial and final input rates.

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