**1. Introduction**

The transient electromagnetic (TEM) method is a classic geophysical exploration method that detects the induction response of a geological body to emitted electromagnetic (EM) waves. It distinguishes subsurface geological structures based on characteristic differences in the amplitude and decay rate of the response of geological bodies with different resistivities. Due to the non-destructive propagation of EM waves in the subsurface medium, TEM is widely used in mineral resource exploration, geological surveys, and urban disease exploration. The TEM signal is characterized by high amplitude in the early time and low amplitude in the late time. Therefore, in order to obtain a high-quality signal, it is necessary to use a wide frequency band, a large dynamic range amplifier, and a high-precision acquisition system with high speed to collect high-quality TEM data for subsequent data processing and data interpretation [1,2].

The observed signal is always interfered with by strong noise in practice. The noise contaminates the signal, especially the late time, which is in low amplitude and represents the deep geological information [3]. These kinds of noise are classified by their source. Part of the noise is from the EM interference in the environment, which is called environmental noise; the other part of the noises is from the TEM receiver, which is called the system noise floor. Environmental noise has a certain pattern in statistics and can be eliminated by using algorithms such as half-cone gate filtering, minimum noise separation, and improved algorithms based on temporal correlation [4–6]. However, these data processing methods cannot eliminate the effect of the system noise floor, so the TEM receiver needs to be optimized to obtain a more accurate signal.

**Citation:** Wang, S.; Zhao, Y.; Sun, Y.; Wang, W.; Chen, J.; Zhang, Y. Design of a Differential Low-Noise Amplifier Using the JFET IF3602 to Improve TEM Receiver. *Micromachines* **2022**, *13*, 2211. https://doi.org/10.3390/ mi13122211

Academic Editors: Lu Zhang, Xiaodan Pang, Prakash Pitchappa and Hae-Jin Kim

Received: 30 September 2022 Accepted: 12 December 2022 Published: 13 December 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

The amplifier noise floor of the TEM receiver is a major part of the system noise floor, and there have been many related studies on TEM low-noise amplifiers (LNA) [7]. Chen et al. designed an amplifier with a noise floor of 1.83nV/√Hz using the low-noise integrated operational amplifier (IOA) AD797 for the ZTEM receiving device [8]. Pi et al. used the IOA LT1028 to design a low-noise preamplifier circuit with an ideal noise of 2.45nV/√Hz for urban TEM devices [9]. Low-noise IOA is mainly used in TEM preamplifiers currently [10,11], but the noise characteristics of IOAs are inferior to discrete components due to the cost restriction, which also limits the noise optimization of subsequent circuits [12]. To solve the problem, Wang et al. developed a 2nV/√Hz ZTEM signal conditioning circuit using the junction field-effect transistor (JFET) LSK389B in the helicopter TEM receiver, which introduced a new idea for the optimization of TEM devices [13].

At present, JFET LNA is mainly used for the measurement and amplification of weak signal sensors such as piezoelectric accelerometers, wireless radio frequency equipment, and seismic accelerometers at present [14]. Scandurra proposed a feedback compensation JFET differential amplifier circuit for low-frequency noise measurement circuits with the noise floor of 1.00nV/√Hz@1.00kHz [15]. Cannat<sup>à</sup> designed a 0.80nV/√Hz@1.00kHz single-ended amplifier based on discrete components JFET for low-frequency noise measurement [16]. To summarize, JFET has a very low system noise floor and a wide stable noise frequency band [17], which is very suitable for noise optimization of TEM receivers.

The rest of the paper is as follows: In Section 2, we analyze the time-frequency characteristics of the TEM signal according to the TEM principle and forward response, and clarify the requirements; Section 3 gives the JFET LNA for TEM receiver with the circuit structure, model, and actual noise floor test; In Section 4, we conducted laboratory experiments to compare the length of TEM effective data from the JFET receiver and the IOA receiver. Finally, the optimization direction of the amplifier and the application effect in transient electromagnetic systems are discussed.
