High-frequency GIC filters using integratedtransconductance-type active devices

Prof. L.T. Bruton, Ph.D., Fel.I.E.E.E., Prof. J.W. Haslett, Ph.D., Sen.Mem.I.E.E.E., and M.K.N. Rao, Ph.D.

Indexing terms: Active filters, Monolithic nullor

Abstract: A transconductance amplifier has been designed and fabricated in monolithic form for use inimplementing high-frequency active filters. The transconductance amplifier, known as a monolithicnullor, is a 3-terminal gain block that may be used to make very high-quality generalised immittance con-vertors. Simulated-inductance and FDNR active ladder filters have been implemented with this amplifierand are superior to the equivalent operational-amplifier derived active filters.

1 Introduction and review

The high-frequency performance of conventional operationalamplifier (op amp) derived active filters is primarily limited bythe finite gainband-width product (GBP) of the op amps[1—3]. At high frequencies, a typical economy 741-typeop amp exhibits a voltage gain frequency responseA(joS) — —B/jcj where B — 2ir x 106 rad/s. Such op ampshave been used to realise high-quality fDNR and simulated-inductance active ladder filters having passband frequenciesup to 100 kHz [2], with a number of authors reporting20-24kHz channel filter designs [3, 4] .

We have implemented a monolithic nullor device usingstandard bipolar microelectronic technology, and have shownthat it may be used to obtain voltage amplifiers with GBPs inexcess of 300 MHz and with a very low spurious phase shift

monolithicnullor

Fig. 1 Ideal nullor model of monolithic nullor

• n r

Fig. 2 2-nullor generalised immitance convenor

0.75 1.56Ho3.6

[5] from input to output. The monolithic nullor is ideallyequivalent to a 3-terminal nullor shown in Fig. 1 (i.e. withnullator and norator sharing a common terminal) andtherefore does not lend itself directly to the realisation ofAntoniou-type GICs [3]. We have therefore proposed the2-nullor GIC configuration in Fig. 2 as the basis for ourdesigns, where the monolithic nullor device is described indetail in Reference 5. A die size of 80 x 80 mil2 (2 x 2 mm2)has been used, with every expectation that this could besignificantly reduced in a final production device. We haveshown that the monolithic nullor GIC may be used toimplement stable FDNR and simulated-inductance elementswith Q-factors that exceed 4000 at 50 kHz and with a ' Q / o 'product better than 200 MHz at frequencies between 20 kHzand 1 MHz. Theoretical studies [5] of the monolithic nullor-derived FDNR and simulated-inductance circuits have shownthat this superior high-frequency behaviour is explained by ahigh-frequency dependence of Q-factor on co"1, whereas forconventional op amp GIC circuit, the Q-factor is proportionalto co~3. We have reported [5], for example, a high-quality 5th-order FDNR lowpass elliptic filter design with a cutofffrequency of 0.5 MHz, good stopband performance to 5 MHzand a passband ripple that deviates by less than ± 0.5 dB overthe temperature range 0-70°C, and by less than ±0.1dB'when monolithic nullors are randomly replaced by othersample devices.

2 High-quality channel-type bandpass simulatedinductance filter

We now report an extension of the practical results achievedwith the new monolithic nullor device. The 12th-orderChebyshev bandpass LC prototype channel filter shown inFig. 3 has a passband from 48.19 kHz to 51.61kHz with

C112.0

0.24,

0.36

0.235,

0.95

0.232,

0.082 4.06 5.8

<-2 L 3 •-/,

Fig. 3 12th-order Chebyshev 48-52 kHz bandpass LC filter

Units in kil, mH and nF for R, L and C, respectively

Paper 1447G, first received 10th April 1981The authors are with the Department of Electrical Engineering,The University of Calgary, 2500 University Drive NW, Calgary,Alberta, Canada T2N 1N4

0.1 dB ripple. (The design details appear in Reference 6.)All inductors are grounded and Q of the highest pole-pair ofthe transfer function is approximately 430, implying thatinductors L2 to Ls must have Q-factors better than about4000 over the passband.

Selecting ^2,3,4,6 =^2,3,4,6 and r s = sCs in Fig. 2 and

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Table 1: Details of active inductors for BPF

Inductors

^-2 ,3 ,1 ,5

Nominal values of GIC elements

R2

520 n

10.68 kfi

* 3

51 on

7.5 ktt

510 n

7.5 ktt

6.8 nF

1.8 nF

39012

2.4 kn

Properties of inductors

L,, Q - 2000L6,Q- 5000

L2,Q> 6000L 3 , Q > 6000L4,Q>6000L 5 , | Q | >2000

employing the monolithic nullors, we obtain the requiredsimulated grounded inductance elements Lx to L6. Thenonideal performance of these active inductors may becalculated from the monolithic nullor modelling equations inReference 5 and, by proper choice of the GICRCelements, themagnitudes of their Q-factors have been designed to exceed2000 without any requirements for Q-factor adjustment ortuning and without selection or matching of monolithicnullor devices [6]. The nominal values of the GIC RC elementvalues are given in Table 1 along with the measured nonidealproperties of the active inductors.

The ideal and measured filter responses are shown in Fig. 4.

0-61-

-20

-40

-60

-80

expandedpassbandresponses

— passband——

-6.0ci

-6.2i

- 64 '

-6.6

46 48 50 52frequency , kHz

54

Fig. 4 Detailed magnitude/gain response characteristics of bandpassfilter

measuredideal

It is observed that the measured response follows the idealresponse very closely. A lateral shift, to the left, of themeasured response is apparent and this shift is the result of thepresence of stray capacitances (— 1 pF) which appear acrossCi —Cs, as confirmed by theoretically computed responses.Over the passband, the almost constant difference of 0.2 dBbetween the ideal and measured responses is partly because ofthe finite Q-factors of the active inductors L2 —Ls and partlybecause of the above-mentioned stray capacitances. In order todemonstrate the fact that the active filter exhibits low active

sensitivity, monolithic nullor chips have been randomlysubstituted in the GICs and it has been observed that thedeviation of the passband response is essentially less than0.1 dB, and that the stopband responses are essentiallyunchanged. The entire filter dissipates 1.2W and has adynamic range of 65 dB.

3 Conclusions

The monolithic nullor device, which has been integrated usingstandard bipolar technology, has been shown to be superior tothe conventional 741-type op amp in stringent high-frequencysimulated-inductance and FDNR active filter applications.Analysis and measurement confirm that the Q-factors of theGIC elements are well behaved at high frequencies. A basicprinciple involved in the design of the monolithic nullor is tomaximise bandwidth by means of voltage/current conversion,as opposed to voltage amplification. It is envisaged that othertypes of active filters, such as switched-capacitor filters andsingle-amplifer filters, will lend themselves to implementationusing integrated voltage-controlled current sources.

4 Acknowledgments

This work was financially supported by the Natural Sciences &Engineering Research Council of Canada under grants A 7776and A7997, and via a Killam Memorial Scholarship to M.K.N.Rao.

5 References

1 BRUTON, L.T.: 'Multiple-amplifier RC-active filter design withemphasis on GIC realizations', IEEE Trans., 1978, 25, pp. 830-845

2 SARAGA, W., HAIGH, D.G., and BARKER, R.G.: 'Microelectronicactive-flC channel bandpass filters in the frequency range 60 -108 kHz for FDM SSB telephone systems', ibid., 1978, 25, pp.1022-1031

3 ANTONIOU, A., and NAIDU, K.S.: 'A compensation technique fora gyrator and its use in the design of a channel-bank filter', ibid.,1975, 22, pp. 316-323

4 BRUTON, L.T., LIM, J.T., and VALIHORA, J.: The feasibility ofactive filtering in frequency-division multiplex system'. ProceedingsIEEE ISCAS, San Francisco, CA, 1974, pp. 121-125

5 HASLETT, J.W., RAO, M.K.N., and BRUTON, L.T.: 'High-frequencyactive filter design using monolithic nullors', IEEE. J. Solid-StateGrcuits, 1980, 15, pp. 955-962

6 RAO, M.K.N.: 'Monolithic VCCS for high-frequency /?C-activecircuits'. Ph.D. thesis, University of Calgary, Calgary, Canada, 1980

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