Design of a Set of Analog Microcircuits for Processing Signals from High-Resistance Sensors

In various areas of instrument engineering, it is necessary to register currents less than 10-12А or process signals from sensors whose internal resistance exceeds hundreds of megaohms. For these tasks, operational amplifiers with input junction field-effect transistors and an n-type channel (n-JFET) are predominantly used. However, there are a number of sensors whose output signal is a short current pulse, for the conversion of which into voltage, charge-sensitive amplifiers are used. The aim of the work was to develop a set of analog microcircuits with an input n-JFET on a master chip, ensuring the processing of signals from various high-resistance sensors. The article presents the electrical circuits of the developed operational amplifiers and charge-sensitive amplifiers, considers their design and circuit features, analyzes the results of circuit simulation, and formulates recommendations for selecting the operating mode of the charge-sensitive amplifier input n-JFET to minimize the rise time and noise level, taking into account operability in the temperature range. As a result of the work carried out, electrical circuits and layout were developed for the master chip of a set of microcircuits with an input n-JFET was created, including the NJAmp1 operational amplifier with parameters close to the AD8625, a low-power NJAmp2 operational amplifier for implementing a voltage follower similar to the AD8244, the NJAmp4 operational amplifier with an input current of less than 5·10-13 А, and a charge-sensitive amplifier allowing adjustment of the drain current of the input n-JFET. Connections of the developed devices according to a typical circuit of an instrumentation amplifier on three operational amplifiers make it possible to obtain a functional analogue of the AD8220 microcircuit.

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