Design of preamplifier circuit for electromagnetic ultrasonic transducer

In nondestructive testing, EMAT is widely used due to its unique advantages. However, the signal received by the EMAT receiving coil is usually very weak, with a small signal amplitude, generally only tens of μV to hundreds of μV, and is highly sensitive to the surrounding noise. The received signal is often submerged in the noise, the radiation pattern is wide, and the energy is not concentrated. In order to obtain a level suitable for display observation, the signal needs to be amplified and filtered to reduce noise and interference. In order to avoid the phenomenon of signal distortion and self-excitation caused by excessive amplification of the EMAT receiving system, multi-stage amplification is usually used. It mainly includes preamplifiers, filters, main amplifiers, and A/D conversion circuits used in digital devices. In order to obtain better results, the preamplifier naturally plays a vital role. Using professional EDA software to simulate and analyze it can analyze the circuit performance more quickly and accurately, so as to select the circuit with better performance and more suitable for the needs. This paper designs two preamplifiers and uses Multisim10 simulation software to simulate and compare these two circuits.

1 Preamplifier

1.1 Amplifier designed with NJM4580

In the first circuit design, the NJM4580 operational amplifier is selected. This amplifier is a dual-channel operational amplifier produced by Japan New Radio Co., Ltd. It has the characteristics of no noise, higher gain bandwidth, high input current and low distortion. It is not only suitable for the audio electronic part and active filter of the audio preamplifier, but also suitable for manual measuring tools, etc.

The main features of NJM4580 are: operating voltage of ±5～±18 V; low input noise voltage of 0.8μV; gain bandwidth of 15 MHz; low distortion of 0.005%; conversion rate of 5V/μV; bipolar technology. The amplifier circuit designed with NJM4580 is shown in Figure 1.

This design uses NJM4580, mainly to maintain the bandwidth of the signal in the differential amplifier circuit design part so that it is not distorted. Three operational amplifiers are arranged in two stages, and the first stage differential amplifier circuit is composed of operational amplifiers U1A and U2A according to the input connection method, and the second stage differential amplifier circuit is composed of operational amplifier U3A. In the first stage circuit, the signal source is added to the in-phase terminal of U1A, and the feedback network composed of R6, R3, and R4 introduces negative feedback.

In order to make the circuit symmetrical and improve the performance of the instrument amplifier, the selected resistor should satisfy the relationship R3=R4, the parameters should be strictly matched, and the error should be controlled within a very small range. After calculation, the output voltage relationship is finally obtained as shown in formula (1):

It can be seen intuitively from formula (2) that the requirements of different signal amplification ratios can be achieved by selecting the proportional relationship between R5/R1 and R3/R6 resistors. Therefore, the selection of resistors is also one of the most important links in the design of instrument amplifiers. Considering the stability and safety of the circuit, the resistance values ​​of R1~R5, R7, and R8 are fixed, and accurate 10kΩ resistors are selected. Only R6 is set to be adjustable. As R6 decreases, the larger the amplification factor, the narrower the bandwidth. Therefore, R6 is determined to be 2 kΩ during the design. This amplifier circuit is a cascade amplifier circuit, which is a pre-stage amplifier, and the post-stage cascade amplifier circuit is composed of two 741 cascades, which together form a complete pre-amplifier circuit at the signal receiving end.

1.2 Amplifier designed using AD620

In the weak signal detection, in order to reduce the interference of the integrated operational amplifier to the circuit, a chip close to the ideal operational amplifier should be selected. It is required to have a small input bias current, input bias voltage and zero drift, and a large common mode rejection ratio and input resistance. Therefore, in another circuit design, AD620 is used to improve the first circuit. AD620 is a high-precision single-chip instrumentation amplifier produced by AD. It has a differential structure and has a strong suppression effect on common mode noise. At the same time, it has a high input impedance and a small output impedance. It is very suitable for amplifying weak signals. In addition, AD620 has good DC and AC characteristics, and has the advantages of low power consumption, high input impedance, low input offset voltage, and high common mode rejection ratio. Its external circuit connection is convenient and simple, and only an external resistor connected to pins 1 and 8 is required to adjust the amplification factor. Gain G = 49.4 kΩ/RG + 1. Among them: RG is the external resistor connected to pins 1 and 8. The main features of AD620 are as follows: bandwidth 800 MHz, output power 24 mW; power gain 120 dB; operating voltage ±15 V; static power consumption 0.48 mW; input offset voltage ≤60 μV; conversion rate 1.2 V/μs; maximum operating current 1.3 mA; input offset voltage 5 μV; input offset drift maximum 1 μV/℃; common mode rejection ratio 93 dB. The circuit designed using AD620 is shown in Figure 2. 

2 Simulation using Multisim 10 software

2.1 Software Introduction

Multisim 10 is launched by National Instruments (NI). Compared with Multisim 10 simulation software, it has more intuitive and user-friendly features, providing more than 16,000 high-quality analog and digital components; various analysis methods (DC sweep analysis, parameter sweep analysis, etc.); voltmeters, ammeters and multiple instruments (digital multimeters, function signal generators, etc.). Most of the software uses actual models to ensure the authenticity and practicality of simulation and experimental results. Multisim 10 can be used to simulate analog circuits, digital circuits, analog-digital hybrids and radio frequency circuits. Among them, its high-frequency simulation and related environment are not available in many general simulation circuit software. This paper designs a voltage signal amplification at the μV level. Two schemes are adopted to compare the performance of the two circuits through the simulation of Multisim 10.

[page]

2.2 Simulation comparison

(1) Setting of function signal generator. Open the signal generator in the software. Since the signal frequency range used in this paper is generally 25 kHz to 1 MHz, in order to simulate the signal received by the sensor, a sine wave signal with an input signal frequency of 100 kHz and an amplitude of 100 μF is selected for analysis and comparison. The function generator settings are shown in Figure 3.

(2) Circuit amplitude-frequency characteristics simulation and comparison. The Bode Plotter in this software is used to simulate and compare the amplitude-frequency characteristics of the two circuits. The observation frequency range is set to 25 kHz to 1 MHz. The results are shown in Figure 4.

Through the Bode plot, we can directly observe that when the input signal frequency is 25 kHz, the gains of the two circuits are 85 dB and 98 dB respectively. By comparison, it can be concluded that the amplification effect of the circuit improved by using AD620 is better. By moving the cursor column of the Bode plotter, we can observe the amplification gain of the two circuits at other frequencies. By moving the cursor to 100kHz, we can directly observe that the gains of the two circuits at this frequency are 60dB and 72dB respectively.

Similarly, by moving the cursor bar, the gains of the two circuits when the input signal is at other frequencies can be obtained. The gains at different input signal frequencies are shown in Table 1. By comparison, it can be seen that the gain of the AD620 circuit is higher than that of the NJM4580 circuit.

(3) Comparison of output signal waveforms. Open the oscilloscope in the software and set it up. Red represents the input signal and green represents the amplified output signal. Select a signal with a frequency of 100 kHz and an amplitude of 100 μV. After amplification by the circuit, the output waveforms are shown in Figure 6. The output waveforms of the two circuits can be clearly seen through Multisim 10 simulation. In order to facilitate the observation of the waveforms, set Channel A (input signal channel) to 100 μV/Div, Channel B (output signal channel) in Figure 6 (a) to 100 mV/Div, and Channel B (output signal channel) in Figure 6 (b) to 500 mV. It can be seen from the waveform that when the input signal is 100 μF, the output signal sizes of the two circuits are 100 mV and 380 mV respectively. Obviously, the improved circuit 2 using AD620 has a larger amplification factor. Through this method, the output waveforms when the input signal is other frequencies can be compared.

3 Conclusion

In this paper, for the input signal with a micro-amplitude level, a cascade amplifier circuit composed of NJM4580 operational amplifier and 741 is designed, and on this basis, AD620 is used to improve the circuit to achieve better performance; Multisim 10 is used to simulate and compare the two designed amplifier circuits, which verifies that the amplifier circuit using AD620 is not only simple in circuit structure, but also has better amplification performance than the differential cascade amplifier circuit composed of NJM4580 operational amplifier.