Design of Electronically Controllable Multifunction Active Filter with Amplitude Controllability Using Two Commercially Available ICs

Onanong, Narumol; Supavarasuwat, Piya; Angamnuaysiri, Daungkamol; Siripongdee, Surapong; Chaichana, Amornchai; Jaikla, Winai; Suwanjan, Peerawut

doi:https://doi.org/10.1155/2022/4932284

Journal of Electrical and Computer Engineering

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Abstract Introduction Analysis Conclusions Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2022 | Article ID 4932284 | https://doi.org/10.1155/2022/4932284

Design of Electronically Controllable Multifunction Active Filter with Amplitude Controllability Using Two Commercially Available ICs

Narumol Onanong,¹Piya Supavarasuwat,¹Daungkamol Angamnuaysiri,²Surapong Siripongdee,¹Amornchai Chaichana,¹Winai Jaikla,¹and Peerawut Suwanjan¹

Academic Editor: Raj Senani

Received12 Apr 2022

Revised13 Jun 2022

Accepted16 Jul 2022

Published04 Aug 2022

Abstract

In this contribution, the design and analysis of an active multifunction biquad filter that provides five voltage-mode filtering configurations in the same core filtering circuit are presented. The design emphasizes using the commercially available ICs, LT1228, which are easy for off-the-shelf implementation and cheaper compared with the chip implementation. The proposed multifunction filter is realized from two commercial LT1228 ICs as the active function block, combined with five passive elements (three resistors and two capacitors) with three input voltage nodes and a single output voltage node. The following advantages are given for this design: (i) it provides high-pass (HP), low-pass (LP), band-stop (BS), band-pass (BP), and all-pass (AP) filtering functions; (ii) orthogonal and electronic tuning of the natural frequency and bandwidth (the quality factor: Q); (iii) output voltage node of the proposed circuit is low impedance; (iv) passband voltage gain is controllable; and (v) matching condition and extra double gain voltage-mode amplifier are not required. The effect of the parasitic element in LT1228 on the filter performance is analyzed and included to strengthen the design idea. The PSpice simulation results using LT1228 with ±5 V and the experimental results tested from the hardware implementation are given to prove the validity of the designed multifunction filter.

1. Introduction

Filters are one of the most important circuits in analog signal processing systems. They are used extensively in many fields, such as electronics, communication systems, control systems, measurement systems, and instrumentation [1]. It can be said that most of the electronic devices in the above works have filters as part of the circuit. For this reason, the filter circuit is continuously developed, researched, and implemented. In particular, a frequency filter that can provide several responses or multiple functions in the same circuit structure has received a lot of attention [2–6]. It is called a multifunction filter circuit. The design of an analog active multifunction filter in the category of multiple-input single-output (MISO) type continues to gain popularity [7, 8]. This is due to the filtering functions, high-pass (HP), low-pass (LP), band-stop (BS), band-pass (BP), and all-pass (AP), which can be selected by controlling on or off the input signals at the input voltage nodes. This feature can be controlled by the digital approach by employing a microcontroller or microprocessor. The voltage-mode analog active filters are always designed to obtain a low-output impedance feature, which makes it convenient to connect with other circuits without the need for buffers. In addition, the filtering parameters should be able to control by an electronic method. Nowadays, electronic devices tend to use a microcontroller or microprocessor to control all functions of these devices. Nevertheless, the MISO voltage-mode filtering topology necessitates reconnection of the inputs once a specific input must be grounded, and another port must be disconnected from ground to be used as a new voltage input. Solely voltage-mode filtering solutions with a single input and a single output (SIMO) can be referred to as “no-reconnect” solutions. In exchange, a low-impedance node lacks a voltage response.

Electronic circuit design using active building blocks (ABBs) is very popular. As it provides a great deal of convenience and flexibility for designers, the circuits designed using active building blocks are simple in structure and often contain a few passive devices [9–11]. Currently, there are a variety of active building blocks with bipolar junction transistor structures or C-MOS transistor structures. Its advantage is that it can operate at a low power supply and low power consumption. In addition, the transistor level-based circuits can work at high frequencies as well. However, the circuits realized at the transistor level are most efficient when they are fabricated into an integrated circuit. The cost of fabricating an integrated circuit into a chip is also expensive. For these reasons, it is always found that the performance of the circuits realized at the transistor or C-MOS level will be verified through simulation only due to cost reasons. In addition, the implementation of circuits into the chip should be manufactured in large quantities to achieve break-even. Therefore, using the commercially available active building blocks to realize the new active circuits with a few amounts for specific applications is more interesting and cheaper. It can be easily purchased commercially ICs in the electronic market. It is also easy to implement cheap circuits for real performance testing [12–14]. An interesting commercial active building block is the LT1228. An LT1228 has an internal structure that consists of the operational transconductance amplifier (OTA) and the current feedback amplifier (CFA). So, it can be used in a variety of applications, such as being used in voltage-mode circuit design, where the output terminal is low impedance, and the input terminals are high impedance. It is also electronically controllable, making it easy to control circuit operations with a microcontroller or microprocessor.

According to the study, the MISO multifunction active filters employing various active building blocks have been presented [15–44]. However, those studies may still have some disadvantages. For example, the ABB is not easily realized from the commercially available ICs [15, 22, 26, 27, 29–34, 36, 40–43]; the filtering parameters are not controlled electronically [17–21, 24–32, 34, 35]; the output impedance node is not low [15, 16, 18–23, 29, 30, 32–37, 40–44]; the matching condition is needed for selecting the filtering functions [20, 28, 31–34, 42, 43]; the extra double gain amplifier is needed [33, 41, 43, 44], and the performance of the proposed multifunction active filters is verified through simulation only [15, 16, 19, 22, 24, 26, 27, 29–34, 36, 37, 40–44]; the quality factor cannot be controlled without affecting the natural frequency [15–38, 40–42, 44]; and the passband gain of the proposed filters cannot be controlled [15–30, 32–44]. The comparison of the proposed commercially available IC-based MISO multifunction active filter and others is given in Table 1.

For the reasons presented above, a commercially available LT1228 IC-based multifunction active filter is proposed in this research paper. The rest of this research contribution is prepared as follows: the basic concept of the commercially available LT1228 IC and the proposed voltage-mode MISO multifunction active filter are given in Section 2, while Section 3 presents, in detail, the analysis and study of the effect of the parasitic impedances in LT1228. The simulation results obtained from the PSpice program are depicted in Section 4. In Section 4, the experimental results obtained from the real circuit test are shown and discussed. Finally, the conclusion of this study is depicted in Section 5.

2. Principle of the Proposed Circuit

2.1. Active Building Block and Its Properties

The commercially available LT1228 IC is a voltage input and current/voltage output active building block that consists of a cascading connection of the transconductance amplifier and the current feedback amplifier [45, 46]. It has five terminals where and are the high-impedance (ideally infinity) voltage input terminals. The y is a high-impedance (ideally infinity) current output terminal of the transconductance amplifier, and this terminal is also the voltage input terminal of the current feedback amplifier section. The low-impedance (ideally zero) terminals x and in LT1228 are the voltage input and output of the current feedback amplifier, respectively. The LT1228 electrical circuit symbol is depicted in Figure 1(a), while the equivalent circuit of LT1228 is shown in Figure 1(b). The pin configuration of the LT1228 is an 8-pin dual-in-line package IC as shown in Figure 2. The LT1228 terminal characteristics are presented in the following equation [46]:where R_T denotes the transresistance gain of LT1228 and denotes the transconductance gain. For ideal consideration, R_T is an inﬁnite value. of LT1228 depends on the DC bias current I_B as depicted in the following equation [46]:

(a)

(b)

Equation (2) indicates that the LT1228-based circuits are electronically tuned, which makes it easy to control circuit parameters with a microcontroller or microprocessor.

2.2. Proposed Multifunction Active Filter

This design focuses on employing the LT1228 IC to obtain the electronic control of the natural frequency and bandwidth (the quality factor), which will be easy for off-the-shelf implementation and cheaper compared with the chip implementation. The presented multifunction active filter is depicted in Figure 3. It contains two LT1228 ICs (LT1228-1 and LT1228-2) as the main active building blocks with three resistors (R, R_a, and R_b) and two capacitors (C₁ and C₂). It has three input voltage nodes, , , and , and a single-output voltage node, . Considering the proposed filter in Figure 3, the output voltage node, , is at the terminal of LT1228-2, which is low impedance. So, the output voltage node of the proposed active filter designed in Figure 3 can be directly connected to the next stage without using any voltage buffers. The input voltage node, , is also high impedance. However, the input voltage nodes, and , are not high impedance. Moreover, some passive elements in the proposed circuit are floating, which can be considered as the drawback when it is compared to the MISO filters proposed in [15, 27, 30, 38, 39, 43]. Considering the ideal properties of the LT1228, the equation of of the proposed circuit is obtained as follows:

Based on the denominator of equation (3), the natural frequency (ω₀) of the active filter designed in Figure 3 is obtained as follows:

Subsequently, the quality factor (Q) of the proposed multifunction active filter designed in Figure 3 is obtained as follows:

The passband voltage gain of the proposed multifunction active filter designed in Figure 3 is given as follows:

The filtering parameters, and Q, which appear in equations (4) and (5), indicate that the control of Q can be done electronically via the transconductance, of LT1228-2 without affecting . Moreover, can be electronically adjusted via the transconductance, of LT1228-1. For practical design, should be first tuned electronically by , and then, the bandwidth or the Q should be electronically adjusted through . In addition, it is observed from equation (6) that the passband gain of all filtering functions can be controlled via the resistor, R_a or R_b, without affecting and Q. So, if the input magnitude signal driven to the proposed multifunction active filter is at a low level, the amplitude of the output voltage can be controlled without using any additional amplifiers.

Considering the output voltage depicted in (3), five second-order filtering functions can be achieved as follows:(i)The band-pass biquad function can be realized from the proposed active filter designed in Figure 3 by driving the input voltage signal to node and connecting nodes and to the ground(ii)The low-pass biquad function can be realized from the proposed active filter designed in Figure 3 by driving the input voltage signal to node and connecting nodes and to the ground(iii)The high-pass biquad function can be realized from the proposed active filter designed in Figure 3 by driving the input voltage signal to node and connecting nodes and to the ground(iv)The band-stop biquad function can be realized from the proposed active filter designed in Figure 3 by driving the input voltage signal to nodes and and then connecting node to the ground(v)The all-pass biquad function can be realized from the proposed active filter designed in Figure 3 by driving the input voltage signal to nodes and and driving the inverting input voltage signal to node . An inverting amplifier with unity gain is required for the all-pass function.

It is found from the above statement that the presented multifunction active filter designed in Figure 3 can provide five responses, named band-pass filter, low-pass filter, high-pass filter, band-stop filter, and all-pass filter, in the same circuit without the need for the double gain amplifier and the passive element matching condition. Moreover, the selection of the output voltage-mode filtering functions can be done by the digital approach employing a microcontroller or microprocessor.

In the case of the all-pass function, the phase response is given as follows:

Equation (7) indicates that the phase difference in the input and output waveform for the AP function is shifted from at low frequencies to at high frequencies with a flat amplitude response where the phase of the output voltage waveform lags the phase of the input voltage waveform.

3. Analysis and Study of the Parasitic Effects on the Proposed Filter

In this section, the effect of the LT1228 parasitic element on the performance of the presented voltage-mode versatile active filter designed in Figure 3 is analyzed and studied. The commercially available LT1228 IC with the parasitic elements at the LT1228 input and output terminals can be drawn in Figure 4. The capacitance and resistance connecting in parallel configuration appear at the high-impedance input and output terminals V₊ (R₊//C₊), V₋ (R₋//C₋), and y (R_y//C_y) and the transresistance impedance R_T//C_T. The resistor and series at the low-impedance terminals x and , respectively. The parasitic effect appeared in the proposed filer is depicted in Figure 5, where Y_A = s(C₊₁ + C₋₂ + C_y1 + C_y2)+G₊₁ + G₋₂ + G_y1 + G_y2, Y₋₁ = sC₋₁ + G₋₁, Y_T1 = sC_T1 + G_T1, and Y_T2 = sC_T2 + G_T2. The frequency limitations at low frequencies of the proposed filter (especially for the BP and HP functions) mostly stem from the parasitic resistances at the first LT1228. Ideally, the first LT1228, R, and C₁ are constructed as the lossless inductance simulator. However, the parasitic resistances cause this sub-circuit to work as the lossy inductance simulator (series LR circuit). If i_y1 is the current going through the lossy inductance simulator (Figure 5), then the relationship between i_y1 and and is as follows:whereand

From equation (10), the frequency limitations at low frequencies of the proposed filter (especially for the BP and HP functions) stem from the resistance, R_eq. To reduce this effect at low frequencies, the value of the external resistor, R, should be low. However, if R_T1 and R₋₁ are greater than r_x1, _, and R, then R_eq 0 and L_eq C₁R/. The effect of the parasitic resistances at the first LT1228 is negligible (we also want to shorten the equation of the output voltage too).

At high frequencies, there are two poles/zeros (we will consider them as the poles) that appear at the inductance simulator. If is more less than R₋₁ and r_x1 is more less than R_T1, these pole frequencies are f_p1 1/(2C₋₁) and f_p2 1/(2C_T1r_x1). The CFA in the second LT1228, R_a, and R_b is constructed as the voltage amplifier. If R_T2 is greater than r_x2, , and R_a, the bandwidth of the amplifier is determined by 1/(2C_T2R_b) [47]. From the simulation in [47], it is indicated that C_y = 6 pF, R_y = 8 MΩ, C− = C₊ = 3 pF, RT = 197.66 kΩ, CT = 5.95 pF, rx = 46.92 Ω, and = 19.80 Ω [47]. If R_b and C₁ used in the proposed circuit are 1 kΩ and 1 nF, the pole frequencies, f_p1 and f_p2, and bandwidth of the amplifier are approximately 2.68 GHz, 570.09 MHz, and 26.75 MHz, respectively. Thus, if the bandwidth of the proposed filter is less than 26.75 MHz, the effect of the parasitic elements, C₋₁, R₋₁, C_T1, R_T1, _, and r_x1 at the first LT1228 and R_T2, C_T2, r_x2, and at the second LT1228, is neglected. So, the most effect is caused by the parasitic elements, C+1, R+1, Cy1, Ry1, C−2, R−2, Cy2, and Ry2. Thus, the output voltage of the presented multifunction active filter with parasitic elements is obtained bywhere C_A = C₊₁ + C₋₂ + C_y1 + C_y2 and G_A = G₊₁ + G₋₂ + G_y1 + G_y2. From equation (11), the natural frequency with the parasitic effect is given as follows:

Subsequently, the quality factor with the parasitic effect is given as follows:

It is observed that the parasitic elements in the LT1228 affect the passband gain, phase response, natural frequency, quality factor, and the operating limitations at low and high frequencies. As discussed above, the value of the external resistor, R, should be low to reduce the parasitic effect at low frequencies. Also, the external resistor R_b should be low to enhance the bandwidth of the proposed circuit. In addition, to control the passband voltage gain with constant bandwidth, the value of R_a should be adjusted [47].

4. Experiment and Simulation Results

The proposed filter is verified with the hardware setup as depicted in Figure 6. The experiment is carried out using the supply voltages ±5 V. The filter is designed to obtain f₀ = 200 kHz, Q = 1, and the passband G_p = 2 by choosing I_B1 = 157.91 µA, I_B2 = 125.66 µA, R = R_a = R_b = 1 kΩ, and C₁ = C₂ = 1 nF. The magnitude frequency response of the proposed active filter for the functions, LP, HP, and BP filter, obtained from the experiment is illustrated in Figure 7. The experimental result shows f₀ = 199.53 kHz (0.23% error) and Q = 1.05 (5% error). The passband voltage gains of the proposed active filter for the functions, LP, HP, and BP filter, obtained from the experiment are 5.95 dB (1.16% error), 5.92 dB (1.66% error), and 5.79 dB (3.82% error), respectively. The magnitude and phase frequency response of the band-stop filtering function obtained from the experiment is depicted in Figure 8. The experimental passband voltage gains of the band-stop function at low and high frequencies are 5.95 (1.16% error) and 5.92 (1.66% error), respectively. The error of the experimental filtering parameters is caused by the parasitic resistances and capacitances in the commercial LT1228 IC as analyzed and discussed in Section 3. The gain and phase frequency behavior of the AP function obtained is depicted in Figure 9. The inverting unity gain amplifier is implemented by AD844 and two resistors (0.47 kΩ). It is seen that the phase behavior of the AP function is shifted from at low frequencies to at high frequencies with a flat amplitude response. The measured sinusoidal input and output waveform of the BP filter is illustrated in Figure 10, where the frequency of the sinusoidal input voltage is 200 kHz @ 50mVp-p. It is found that the sinusoidal input and output voltage signals are in phase at the center frequency (f = 200 kHz), while the BP output voltage is 98.7 mV_p-p, which is around twice as large as the input. The total harmonic distortion (THD) of the BP output voltage is 0.191%. As expected in equation (5), the control of the Q is done without affecting the f₀ by setting I_B2 to 37.67 µA, 66.75 µA, and 125.66 µA (I_B1 = 157.91 µA). This advantageous feature is confirmed by the experimental result in Figure 11. The experimental quality factors obtained from these values of I_B2 are, respectively, varied to 2.69, 2.15, and 1.05. From the experimental result in Figure 12, changing I_B1 to 79.34 µA, 125.66 µA, and 304.90 µA (I_B2 = 125.66 µA) results in the natural frequency changing to 144.54 kHz, 199.53 kHz, and 288.40 kHz, respectively. However, adjusting I_B1 will also affect the quality factor. So, in the practical design step, the natural frequency should be first tuned electronically by I_B1, and then, the bandwidth or the Q is electronically adjusted through I_B2. As mentioned in Section 3, the frequency limitation at high frequency of the proposed filter depends on the resistor, R_b. So, to control the passband voltage gain with constant bandwidth, the value of R_a should be adjusted [47]. The control of passband gain is experimentally verified via the BP function by adjusting the value of R_a to 0.5 kΩ, 1 kΩ, and 2 kΩ, respectively (R_b = 1 kΩ), as depicted in Figure 13. The experimental passband gains for these R_a values are 9.30 dB (2.52% error), 5.79 dB (3.82% error), and 3.27 dB (7.10% error), respectively. It is found that at a high value of R_a, the voltage gain error is quite high. So, the R_b can be decreased (R_a must be decreased too) to get a higher voltage gain. Also, if R_b is set to a low value, it can make the proposed circuit’s bandwidth wider. The experimental result confirms that the passband gain is controllable as expected in equation (6). The step response of the low-pass filter is shown in Figure 14 for an input step amplitude of 50 mVp-p (f = 15 kHz).

In addition, the PSpice software is used to evaluate the performance of the proposed multifunction active filter. The macro-model of PSpice is at level 3. The input dynamic range of the proposed filter is evaluated by testing the THD of the output voltage versus the amplitude of the input voltage for band-pass filtering functions. Figure 15 depicts the dependence of THD on the amplitude of the sinusoidal input voltage signal obtained through simulation, where the frequency of the sinusoidal input signal was 200 kHz. When the input voltage is less than 180 mV_p-p, the THD of the band-pass filtering function falls below 1 percent. The band-pass function of the proposed multifunction filter is analyzed using the Monte Carlo method to examine the influence of passive elements on the performance of the circuit. In this simulation, the Gaussian variation of the resistance and capacitance is 5%, while the other active elements are identical to those observed in the previous experiment. Statistical analysis is performed on the Monte Carlo 100 simulation. After 100 runs, the band-pass filter’s center frequency varies between 186.26 kHz and 207.93 kHz, with a standard deviation of 4247.02 Hz as depicted in Figure 16.

5. Conclusions

In this design, the voltage-mode versatile biquadratic filter with amplitude controllability using the commercially available LT1228 ICs is proposed. The proposed filter contains two LT1228 ICs, three resistors (R, R_a, and R_b), and two capacitors (C₁ and C₂). The low-impedance output voltage node of the proposed circuit is achieved, which is able to connect to other circuits without the need for external buffer circuits. Five standard filter responses, including HP, BP, BS, LP, and AP filtering functions, can be obtained in the same core filtering structure without any matching conditions and a double gain amplifier. With this advantage feature, the filtering function of the proposed versatile active filter can be chosen by the digital approach. Both parameters (f₀ and Q) can be electronically adjusted by I_B1 and I_B2. Moreover, Q can be tuned by the bias current, I_B1, without affecting the parameter, f₀. The passband gain is controllable by adjusting the resistors R_a and R_b without disturbing the parameters, Q and f₀. However, the inverting unity gain amplifier is required for the AP filtering function. The input voltage nodes, and , are not high impedance. The simulation results from the PSpice program and experiment results from the hardware test are carried out to investigate the proposed multifunction active filter. With the supply voltages ±5 V and choosing I_B1 = 157.91 µA, I_B2 = 125.66 µA, R = R_a = R_b = 1 kΩ, and C₁ = C₂ = 1 nF, the natural frequency obtained from the experiment is 199.53 kHz (0.23% error). The quality factor obtained from the experiment is 1.05 (5% error).

Data Availability

No data were used to support the findings of this study.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This work was supported by King Mongkut’s Institute of Technology Ladkrabang (KREF016301).

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Copyright © 2022 Narumol Onanong et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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