Three-Input Single-Output Current-Mode Universal Filter Using a MCDTA

Montree Kumngern, Kulasak Khwama, Somyot Junnapiya Department of Telecommunications Engineering, Faculty of Engineering,

King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand kkmontre@kmitl.ac.th

Abstract—This paper presents a new three inputs and one output current-mode universal filter employing one modified current differencing transconductance amplifier (MCDTA) and two grounded capacitors. The filter can realize low-pass, band-pass, high-pass, band-stop and all-pass responses by appropriately connecting the input terminals. For realize filtering functions, without component-matching conditions and inverting-type input signal requirements. The natural frequency can be electronically controlled by the bias currents of MCDTA. The active and passive sensitivity of the filter are low. The PSPICE simulation results are performed to confirm the presented theory.

Keywords—universal filter, modified current differencing transconductance amplifier, analog signal processing

I. INTRODUCTION The universal filters are the circuits that yield five standard

transfer functions; i.e., low-pass (LP), band-pass (BP), high-pass (HP), band-stop (BS) and all-pass (AP), which plays an important role in the fields of analog signal processing. The filters could be applied in the implementation of phase-locked loop (PLL) frequency modulation stereo demodulation, touch-tone telephone tone decode, cross-over network used in a three-way high-fidelity loudspeaker [1]. Beside, the active filters employing grounded capacitors are beneficial from the point of view of integrated circuit (IC) implementation [2].

Recently, a relatively new current-mode active element with two current inputs and two kinds of current output, which is called a current differencing transconductance amplifier (CDTA), was introduced [3]. This device is a synthesis of the well-known advantages of the current differencing buffered amplifier (CDBA) [4] and the transconductance amplifier to facilitate the implementation of current-mode analog signal processing. Therefore, the CDTA is quite suitable for the synthesis of current-mode filters with electronically tunability property. Moreover, CDTA-based circuit provides the circuit realizations with a reduced number of resistors. As a result, a number of CDTA-based filters have been reported in the technical literature [5]-[8]. However, these reported filters require more than one CDTA.

More recently, a CDTA has been developed next which is called a modified current differencing transconductance amplifier (MCDTA) [9]. The MCDTA is realized by adding a transconductance amplifier and a current mirror in CDTA, and

extending the number of x ports and z ports. The relatively independent of second TA in MCDTA is different from the existing CDTA. This property makes it different from conventional CDTA. Therefore, a MCDTA has one existing TA and one independently TA which is sufficient for realizing multiple inputs and single outputs (MISO) universal filters. A few MCDTA-based universal filters were already proposed [9]-[10]. However, these reported circuits not provide five standards the filter response and classify as a single input and multiple output (SIMO) filter. Compared with the SIMO filters, the MISO filters provide a variety of circuit characteristics with different input and output signals, usually do not require any parameter-matching conditions and also may lead to a reduction in the number of active elements. Moreover, to realize a larger variety of filter functions such as inverting or non-inverting type functions, the multiple-input filters seem to be more suitable than the SIMO filter.

In this paper, a new three inputs and one output current-mode universal biquadratic filter employing only one MCDTA and two grounded capacitors is presented. The proposed circuit can simultaneously realize LP, BP, HP, BS and AP responses by appropriately connecting the input terminals. For realize the filtering responses, no component-matching conditions are required. The natural frequency can be electronically controlled though the bias currents and the quality factor can be given by the ratio of passive components. The active and passive sensitivities of the filter are low. The output impedance terminals of the circuit are also high. The PSPICE simulation results are also performed to verify the characteristics of the proposed filter.

II. CIRCUIT REALIZATION The electrical symbol and equivalent circuit of the MCDTA

are respectively shown in Fig. 1 (a) and (b) [9]. The characteristic of MCDTA can be described by the following set of equations:

p n

z1 1 p n

x1 m1 z1

x2 m2 z2

V V 0

I I I I

I g V I g V

z

2013 Eleventh International Conference on ICT and Knowledge Engineering

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(a)

(b)

Figure 1. MCDTA: (a) electrical symbol, (b) equivalent circuit.

where gm1 and gm2 are the transconductance gain of MCDTA. The properties of this device are similar to the conventional CDTA [3], but the MCDTA [9] has independent TA (gm2). This property makes it different from conventional CDTA.

The proposed current-mode multifunction filter using MCDTA as active element is shown in Fig. 2. It consists of one MCDTA and two grounded capacitors. The use of grounded capacitors makes the proposed circuits ideal for integrated circuit implementation [2].

MCDTA

p

z1

x2+

x2+

x1+

z2 x2-

C2

nIin3

C1

Iin2

Iin1

Iout

Figure 2. Proposed universal filter using MCDTA.

Routine filter analysis using the MCDTA characteristic given in equation (1), the current output Iout of Fig. 2 can be expressed as

2 m2 m1 m2 m2 m1 m2in3 in2 in1

2 1 2 2 1 2out

2 m2 m1 m2

2 1 2

g g g g g gs I - s I + IC C C C C C

Ig g gsC C C

From equation (2), variant filter responses can be obtained as:

(1) The LP: if Iin1=Iin2=0 and Iin3=Iin. (2) The BP: if Iin1=Iin3=0 and Iin2=Iin. (3) The HP: if Iin1=Iin2=Iin3=Iin. (4) The BS: if Iin3=0 and Iin1=Iin2=Iin. (5) The AP: if Iin3=0 and Iin1=2Iin2=Iin.

Therefore, the filter in Fig. 2 can realize five types of standard biquadratic function into one circuit. It should be noted that the realization requires no component-matching conditions and inverting-type input signal requirements.

Figure 3. CMOS implementation of MCDTA.

The parameters o and Q are calculated, respectively, as

m1 m2o

1 2

g gC C

m2 1

m1 2

g CQ=

g C

It is evident from equations (3) and (4) that the parameter o can be tuned by varying the transconductance gain gm (i.e., gm=gm1=gm2) and parameter Q can be given by C1/C2.

The possible CMOS realization of the MCDTA used in this work is shown in Fig. 3. Assume that transistors M9 and M10, in Fig. 3 are identical, the gm1 can be approximately obtained as

m1 n ox b1g C W/L I

Also, assume that transistors, M11 and M12, are identical, the gm2 can be expressed by

m2 n ox b2g C W/L I

The multiple-output plus/minus MCDTA can be obtained by adding additional current mirrors and cross-coupled current mirrors to obtain the provided plus and minus type outputs (±x).

III. SIMULATION RESULTS The proposed filter in Fig. 2 was simulated using PSPICE

simulators. The MCDTA in Fig. 3 was simulated using 0.25 m TSMC CMOS process. The aspect ratios of NMOS and PMOS devices for Fig. 3 were: W/L=2 m/0.5 m for M1-M4, W/L=7 m/0.5 m for M5-M8, W/L=5 m/0.5 m for M9-M12 and all Mn devices and W/L=15 m/0.5 m for all Mp devices. The supply voltage and the bias current were given as

1.5 V and 50 A.

Gai

n, d

B

Figure 4. Simulated frequency responses of proposed LP, BP, HP and BS

filter.

10k 100k 1.0M 10M 100M 1.0G-40

-20

0

20

40

-400

-300

-200

-100

0

Frequency, Hz

Gain, dB Phase, dB

gain

phase

Figure 5. Simulated gain and phase responses of the proposed AP filter.

-50

-40

-30

-20

-10

0

10

10k 100k 1.0M 10M 100M 1.0G

Frequency, Hz

Gai

n, d

B

Ib=20 A

Ib=120 AIb=50 AIb=30 A

Figure 6. Simulated frequency response of BP filter at different bias current.

9.0 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9-20

-15

-10

-5

0

5

10

15

20

10.0

Time, s

Cur

rent

,A

iin

iout

Figure 7. Time-domain input and output signal waveforms of the BP filter.

0

1

2

3

4

5

6

7

0 20 40 60 80 100

THD

, %

Bias current, A (peak-to-peak)

Figure 8. The output THD of BP filter on input current amplitude.

As an example design, C1=C1=10 pF was given. Fig. 4 shows the simulated frequency response of LP, BP, HP and BS filters at Ib1=Ib2=50 A. From Fig. 4, a natural frequency fo of 5.24 MHz and Q 1 was obtained. Fig. 5 shows the simulated frequency responses of the gain and phase characteristics of the AP filter at fo 5.24 MHz. It was clear from Figs. 4 and 5 that the proposed filter performs five standard biquadratic filtering functions well. At the bias currents Ib1=Ib2= 50 A, the power consumption was 2.5 mW. Fig. 6 shows the simulated a BP filter response when the bias currents Ib (i.e., Ib=Ib1=Ib2) were simultaneously adjusted for the values 20, 30, 50 and 120 A, respectively. This simulation result was confirmed by equation (3), where gm1 and gm2 was respectively varied by adjusting the bias currents Ib1 and Ib2. In order to test the input dynamic range of the proposed filter, the simulation was repeated for a sinusoidal input signal at fo 5.24 MHz. Fig. 7 shows that the input dynamic range of the BP response at Ib1=Ib2=50 A and C1=C2=10 pF, extends up to amplitude of 20μA (peak to peak) without signification distortion. The THD of this figure is 2.81 %. The dependence of the output harmonic distortion of BP filter on input current amplitude was summarized in Fig. 8. It can see from Fig. 8 that the THD was about 5.13 % when the input signal was increased to 90 A (peak to peak). The THD value will reduce if lower frequency was supplied.

IV. CONCLUSIONS In this paper, a new three inputs and one output current-

mode universal biquadratic filter employing only one MCDTA and two grounded capacitors was proposed. It possesses the following properties: (i) employment of only grounded capacitors which is suitable for IC implementation; (ii) suitability of implementation in CMOS technology; (iii) ability of realizing the LP, BP, HP, BS and AP filter responses without any component-matching condition requirements; (iv) electronic control of the parameter o and (iv) active and passive sensitivities are low. The simulation results are performed to demonstrate the effectiveness of our schemes.

REFERENCES [1] W. H. Hayt, J. E. Kemmerly, S. M. Durbin, Engineering Circuit

Analysis, New York, McGraw-Hill, 2002. [2] M. Bhusan and R. W. Newcomb, “Grounding of capacitors in integrated

circuits,” Electronics Letters, vol. 3, pp. 148-149, 1967. [3] D. Biolek, “CDTA-building block for current-mode analog signal

processing,” in Proceedings of the European Conference on Circuit Theory and Design (ECCTD’03), Poland, 2003, pp. 397-400.

[4] C. Acar, S. Ozoguz, “A new versatile building block: current differencing buffered amplifier suitable for analog signal-processing filters,” Microelectronics Journal, vol. 30, pp. 157-160, 1999.

[5] A. U. Keskin, D. Biolek, E. Hancioglu, V. Biolkova, “Current-mode KHN filter employing current differencing transconductance amplifiers,” International Jourmal of Electronics and Communications, vol. 60, pp. 443-446, 2006.

[6] W. Tangsrirat, T. Dumawipata, W. Surakampontorn, “Multiple-input single-output current-mode multifunction filter using current differencing transconductance amplifiers,” International Jourmal of Electronics and Communications, vol. 61, pp. 206-214, 2007.

[7] N. A. Shah, M. Quadri, S. Z. Iqbal, “Three input one output current-mode cascadable universal filter employing CDTAs,” Journal of Active and Passive Electronic Devices, vol. 4, pp. 347-352, 2009.

[8] D. Prasada, D. R. Bhaskar, A. K. Singh, “Universal current-mode biquad filter using dual output current differencing transconductance amplifier,” International Journal of Electronics and Communications, vol. 63, pp. 497-501, 2009.

[9] Y. Li, “A modified CDTA (MCDTA) and its applications: designing current-mode sixth-order elliptic band-pass filter,” Circuits, Systems, and Signal Processing, vol. 30, pp.1383-1390, 2011.

[10] Y. Li, “Current-mode sixth-order elliptic band-pass filter using MCDTAs,” Radioengineering, vol. 20, pp. 645-649, 2011.