Invention | Free full text | Design of a new type of permalloy thin film magnetic induction switch and transimpedance amplifier circuit

Among the developed magnetic sensing devices, magnetic sensors based on the AMR effect are widely used due to their high sensitivity, wide temperature range, low power consumption, and low cost. [10,11,12]. The AMR effect is caused by anisotropic scattering of 4s electrons in exchange split 3D orbitals [13].This effect is very obvious in PM films [14,15]. Therefore, a PM film composed of 80% nickel and 20% iron was deposited on the device by magnetron sputtering. The surface roughness was measured using atomic force microscopy (AFM), as shown in Figure 1a.The average roughness is 1.985 nm, qualifying it for further fabrication into planar magnetic devices [8]. The film is then patterned into a strip structure. Due to the shape anisotropy of the stripe pattern, it is easily magnetized along the long axis.In addition, barber pole electrodes [16] is introduced onto the PM film because it can rotate the current direction by an angle of 45° or 135° without a magnetic field ($H_{eXt}$). Therefore, the output voltage exhibits a linear relationship near zero magnetic field. Furthermore, under actual operating conditions, sensors are affected by temperature changes. The output signal of the PM film will change due to its environment, thus reducing the stability and accuracy of the sensor.For this purpose, a Wheatstone bridge (WB) structure is used [2,17]. The WB structure contains 4 branches, each branch consists of one or more PM strips.When the external magnetic field intensity $H_{eXt}$ When changing, WB outputs the sensor’s measurement results in the form of a differential voltage. Through the differential output of the bridge, the measurement error of the PM film caused by external factors can be offset. Figure 1b shows the structure of WB. To show details, Figure 1b shows only half of the structure, where the structures of the permanent tape and barbershop electrodes can be clearly seen.

The AMR sensor is fabricated based on the structure of WB and barber-pole electrodes. To facilitate testing, the sensor chip is packaged as shown in Figure 2a. Only one PM sensor strip is needed to act as the sensing element for the switch in this figure. This reduces power consumption and minimizes the size of the chip. Its structural diagram is shown in Figure 2b. When a current is applied to a permanent magnetic tape together with an external magnetic field, the direction of magnetization (medium size)The internal alloy changes, forming an angle ($I$) and the current direction (J).with changes $I$can change the resistance of the entire alloy.

The sensing device has a certain resistance value at zero magnetic field: ${\mathrm{right}}_0$ and some current value $I_{reF}$both of which are related to the strength of the external magnetic field $H_{eXt}$. medium size Counterclockwise rotation due to increase $H_{eXt}$ direction (A) in Figure 2b, and $I$ Increase until it reaches 90 degrees.minimum resistance ${\mathrm{right}}_{\perp }$ It is achieved in this state. on the contrary, medium size Rotate clockwise due to increase $H_{eXt}$ has direction (B), and $I$ Decrease until it reaches 0.Maximum resistance ${\mathrm{right}}_{\parallel }$ upon arrival J parallel to medium size. $H_{eXt}$ in the opposite direction to make the resistance value higher or lower than ${\mathrm{right}}_0$ and the current value is lower or higher than $I_{reF}$.When the sensing device makes constant rotational motion $H_{eXt}$ with 100 kHz or when $H_{eXt}$ With relative movement in opposite directions (A) or (B), the output of the sensing device becomes a sinusoidal current wave, as shown in Figure 2c. Therefore, switching operation can be achieved by designing a circuit that compares the output current and current. $I_{reF}$.

The schematic diagram of the magnetic induction switch proposed in this article is shown in Figure 3. In addition to the sensing device, a current comparison circuit is also used as a subsequent processing circuit. The final output is a high level or low level signal. This module functions to control the switch on or off through a magnetic field in the opposite direction.

The current comparison circuit is shown in Figure 4a.current signal $I_{In}$ is the current of the applied sensing device $H_{eXt}$.direction $H_{eXt}$ in two opposite directions parallel to the X-axis in Figure 3.Congressman₁MP₂ and minnesota₁minnesota₂ Construct two simple current mirror circuits to replicate $I_{reF}$ and $I_{In}$ To reduce the impact of process and temperature on the circuit. To reduce input impedance and response time, MP₃ and minnesota₃ connected as a source follower and at the output, MP₄ and minnesota₄ Connect as a CMOS inverter to implement positive feedback function. One of the advantages of using low input resistance is to reduce the influence of parasitic parameters of the sensing device at the front end of the circuit.

The transient simulation results of this circuit are shown in Figure 4b.this $I_{reF}$ Approximately 0.42 mA at 100 kHz, supply voltage $V_{dd}$ is 1.8V.The output voltage $U_{\mathrm{oh}\mathrm{you}t}$ Alternate between high and low (1.76 V and 24 mV) $I_{In}$ Variety.according to $U_{\mathrm{oh}\mathrm{you}t}\mathrm{result}$direction $H_{eXt}$ It can be inferred. when. . .when $U_{\mathrm{oh}\mathrm{you}t}$ at a high level and can be proven $I_{In}$ > $I_{reF}$; Therefore, the resistance is less than ${\mathrm{right}}_0$. According to the sensing principle in the previous section, it can be inferred: $\mathrm{this}{H}_{eXt}$ This is the (A) direction. on the contrary, $H_{eXt}$ It is in the (B) direction. Therefore, based on a sensing device and a simple circuit, two magnetic fields in opposite directions can control the switch on or off.

However, the circuit in Figure 4a has obvious disadvantages. First, since the input voltage of the positive feedback inverter does not convert from rail to rail, transistors MN4 and MP4 cannot turn off completely, resulting in additional DC power consumption. In addition, when the input current is very low, there will be a temporary dead zone in the dynamic response, in which both input transistors MN3 and MP3 are turned off. This condition, combined with parasitic effects, causes the input impedance to become very large. This may eventually lead to errors in judgment and inaccurate output voltage signals. At the same time, this circuit cannot meet the needs of high-frequency modules. Therefore, this article uses a transimpedance amplifier circuit to improve the performance of the designed module.