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A novel mixed-mode OTA-C universalfilterMuhammad Taher Abuelma'atti & Abdulwahab Bentrciaa King Fahd University of Petroleum and Minerals, Box 203,Dhahran 31261, Saudi Arabiab King Fahd University of Petroleum and Minerals, Box 203,Dhahran 31261, Saudi Arabia E-mail:Version of record first published: 19 Aug 2006.

To cite this article: Muhammad Taher Abuelma'atti & Abdulwahab Bentrcia (2005): A novel mixed-mode OTA-C universal filter, International Journal of Electronics, 92:7, 375-383

To link to this article: http://dx.doi.org/10.1080/08827510412331295009

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International Journal of Electronics,

Vol. 92, No. 7, July 2005, 375–383

A novel mixed-mode OTA-C universal filter

MUHAMMAD TAHER ABUELMA’ATTI* and ABDULWAHAB BENTRCIA

King Fahd University of Petroleum and Minerals, Box 203, Dhahran 31261, Saudi Arabia

(Received 3 March 2004; in final form 1 June 2004)

A novel mixed-mode biquad circuit is presented. The circuit uses six single-output- and one dual-output-operational transconductance amplifiers, twogrounded capacitors; and can realize lowpass, highpass, bandpass, notch,lowpass-notch, highpass-notch and allpass responses from the same topology.The circuit can be driven by voltage or current and its output can be voltage orcurrent. The parameters !o and !o/Qo enjoy independent electronic tunability.Simulation results are included.

Keywords: Active filters - operational transconductance amplifier

1. Introduction

Due to their structural simplicity, electronic tunability, high frequency capability,and monolithic integrability, active filters using operational transconductanceamplifiers (OTAs) and grounded capacitors, widely known as OTA-C filters, areattracting the attention of many researchers; see for example (Ananda Mohan1990, Sun and Fidler 1993a, b, Ramirez-Angulo and Sanchez-Sinencio 1994, Sunand Fidler 1995, Al-Hashimi 1996, Sun and Fidler 1996a, b, c, d, Tsukutani et al.1996, Moniri and Al-Hashimi 1997, Sun and Fidler 1997a, b, Zhang et al. 1997,Al-Hashimi et al. 1998, Sun et al. 1998, Deliyannis et al. 1999, Lee et al. 1999, Chiangand Schaumann 2000, Glinianowicz et al. 2000, Hsu and Feng 2000a, b, 2001a, b)and the references cited therein. The use of multiple-output OTAs, paves the way torealize current-mode filter functions (see, for example, Ramirez-Angulo andSanchez-Sinencio 1994, Al-Hashimi 1996, Sun and Fidler 1996a, b, c, Tsukutaniet al. 1996, Moniri and Al-Hashimi 1997, Sun and Fidler 1997, Al-Hashimi et al.1998, Lee et al. 1999, Hsu and Feng 2000, 2001), whereas the use of single-outputOTAs yields voltage-mode filter functions (see, for example, Ananda Mohan 1990,Sun and Fidler 1993a, b, 1995, 1996, 1997, Zhang et al. 1997, Sun et al. 1998,Deliyannis et al. 1999, Glinianowicz et al. 2000, Chiang and Schaumann 2000,Hsu and Feng 2000, 2001).

In analog signal processing applications it may be desirable to have activefilters with input currents and/or voltages and output currents and/or voltages,that is mixed-mode filters (Ramirez-Angulo et al. 1992, Soliman 1996, Toker et al.2001). Careful inspection of the available literature shows that the mixed-mode

*Corresponding author. Email: mtaher@kfupm.edu.sa

International Journal of Electronics

ISSN 0020–7217 print/ISSN 1362–3060 online # 2005 Taylor & Francis Group Ltd

http://www.tandf.co.uk/journals

DOI: 10.1080/08827510412331295009

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realizations with input current and output voltage (Ramirez-Angulo et al. 1992,Soliman 1996), and with input voltage and output current (Toker et al. 2001) areavailable, no circuit realization is available for realizing a generalized mixed-modeactive filter with input current or voltage and output current or voltage. It is themajor intention of this paper to present such a generalized mixed-mode circuit usingthe OTAs.

2. Proposed circuit

The proposed circuit is shown in figure 1. Routine analysis yields the transferfunctions given by

Vout ¼gm2

gm4gm7

N sð Þ

D sð Þð1Þ

and

Iout ¼gm2

gm4

N sð Þ

D sð Þð2Þ

where

N sð Þ ¼ s2 gm3Vin3 þ Iin3ð Þ � sgm3

gm5Vin2 � gm6Vin4 þ Iin2ð Þ

C1

þgm3gm5

C1C2

gm1Vin1 þ Iin1ð Þ ð3Þ

•

•

•

•

•

•

•

•

••

•

•

•

•+

+

+

+

+

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+

−

−

−

−

−

−

−

−1mg 2mg

3mg

4mg

5mg 6mg

7mg

1C

2C

1inV

2inV

3inV

4inV

1inI

2inI

3inI

outIoutV

•

Figure 1. Proposed mixed-mode OTA-based filter.

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and

D sð Þ ¼ s2 þ sgm3gm6

gm4C1

þgm1gm3gm5

gm4C1C2

: ð4Þ

Inspection of equations (3) and (4) shows that various mixed-mode filter functions,with input voltage or current and output voltage or current, can be realized.For example:

(1) With Iin1 ¼ Iin2 ¼ Iin3 ¼ 0, the following voltage-mode responses, with inputvoltages and output voltage, and mixed-mode responses, with input voltagesand output current are obtained:(a) a non-inverted highpass-filter (HPF) with Vin1 ¼ Vin2 ¼ Vin4 ¼ 0.(b) a non-inverted lowpass-filter (LPF) with Vin2 ¼ Vin3 ¼ Vin4 ¼ 0.(c) an inverted bandpass-filter (BPF) with Vin1 ¼ Vin3 ¼ Vin4 ¼ 0.(d) a non-inverted BPF with Vin1 ¼ Vin2 ¼ Vin3 ¼ 0.(e) a non-inverted notch filter (NF) with Vin1 ¼ Vin3 and Vin2 ¼ Vin4 ¼ 0.(f) in case (e), a lowpass-notch and highpass-notch can be obtained by

adjusting the transconductances gm3 and gm4.(g) a non-inverted allpass-filter (APF) with gm3 ¼ gm4, gm5 ¼ gm6,

Vin1 ¼ Vin2 ¼ Vin3 and Vin4 ¼ 0.(2) With Vin1 ¼ Vin2 ¼ Vin3 ¼ Vin4 ¼ 0, the following current-mode responses,

with input currents and output current, and mixed-mode responses, withinput currents and output voltage, are obtained:(a) a non-inverted HPF with Iin1 ¼ Iin2 ¼ 0.(b) a non-inverting LPF with Iin2 ¼ Iin3 ¼ 0.(c) an inverting BPF with Iin1 ¼ Iin3 ¼ 0.(d) a NF with Iin2 ¼ 0 and Iin1 ¼ Iin3.(e) In case (d), a lowpass-notch and highpass-notch can be obtained

by adjusting the transconductances gm3 and gm4.(f) an APF with Iin1 ¼ Iin2 ¼ Iin3, and gm1 ¼ gm4 ¼ gm6.

Inspection of equation (4) shows that in all cases the parameters !2o and !o/Qo are

given by

!2o ¼

gm1gm3gm5

gm4C1C2

ð5Þ

!o

Qo

¼gm3gm6

gm4C1

ð6Þ

and

Qo ¼1

gm6

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiC1

C2

gm4

gm3

gm1gm5

sð7Þ

Thus, the parameter !2o can be controlled by adjusting the transconductances

gm1 and/or gm5 without disturbing the parameter !o/Qo, and the parameter !o/Qo

can be controlled by adjusting transconductance gm6 without disturbing the param-eter !2

o. Moreover, the parameter Qo can be controlled by adjusting the trans-conductance gm6 without disturbing the parameter !2

o. However, the parameter!2o cannot be controlled without disturbing the parameter Qo. In addition,

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inspections of equations (1) and (2) shows that the gain associated with any transferfunction can be controlled, without disturbing the parameters !2

o and !o/Qo byadjusting gm2 and/or gm7 for output voltage, or by adjusting the transconductancegm2 for output current. Thus, the proposed circuit enjoys the attractive feature ofindependent electronic tunability of the transfer gain and the parameters !2

o and!o/Qo, and orthogonal tuning of the parameters !2

o and Qo.Using equations (5) and (6), it is easy to show that all the passive sensitivities

of the parameters !o and !o/Qo are lesser than or equal to unity. Thus the circuitparameters enjoy low passive sensitivities.

3. Simulation results

The proposed circuit was simulated using PSPICE circuit simulation program. TheOTAs were modeled using the model reported in (Wu 1994). The results obtainedfrom voltage-mode realization, with C1 ¼ C2 ¼ 1:0 nF, gm1 ¼ gm2 ¼ gm3 ¼ gm4 ¼

gm5 ¼ gm6 ¼ gm7 ¼ 1:0mA=V, are shown in figures 2–5. The results obtained fromcurrent-mode realizations, with C1 ¼ C2 ¼ 1:0 nF, gm2 ¼ gm3 ¼ gm4 ¼ gm5 ¼ gm6 ¼

gm7 ¼ 1:0mA=V, gm1 ¼ 0:1mA=V, are shown in figures 6–9. Figures 2–9 alsoshow calculations made using equation (1). It appears from figures 2–9 that thesimulated and calculated results are fairly in good agreement.

4. Conclusion

In this paper, a new mixed-mode biquad circuit has been presented. The circuit usessix single-output and one dual-output OTAs and can realize all the standard biquad

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Figure 2. Calculated (––––) and simulated (**) voltage-mode bandpass filter.

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filter responses. The parameters of the filter responses enjoy independent electronictunability and low passive sensitivities. The circuit can also realize mixed-modefilter responses. Thus, a voltage input may result in either a current or a voltageoutput. In addition, a current input may result in either a current or a voltageoutput.

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Figure 3. Calculated (––––) and simulated (**) voltage-mode highpass filter.

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Figure 4. Calculated (––––) and simulated (**) voltage-mode lowpass filter.

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In order to confirm the operability of the proposed circuit, the simulation resultsreported in this paper were obtained using capacitor values 1.0 nF. However, forintegrated circuit implementations, these capacitor values are rather large andcapacitors of the order of 10 pF are more appropriate. This may require the

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Figure 5. Calculated (––––) and simulated (**) voltage-mode notch filter.

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Figure 6. Calculated (––––) and simulated (**) current-mode bandpass filter.

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reduction of the transconductance values for the proposed circuit to work withinthe bandwidth of the OTAs.

While the simulation results confirm the theory presented in this paper,the discrepancies between the calculated and simulated results are attributed

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Figure 7. Calculated (––––) and simulated (**) current-mode highpass filter.

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Figure 8. Calculated (––––) and simulated (**) current-mode lowpass filter.

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to the simplified model used in the analysis. More sophisticated models for the OTAsmust be used if operation at high frequencies is targeted.

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Figure 9. Calculated (––––) and simulated (**) current-mode lowpass-notch filter.

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