Implementation of the OTA Circuit Design for Gm-C Active Filter

Implementation of the OTA Circuit Design for

Gm-C Active Filter

Youngmin Kang, Byun-Gon Kim, Kwan-Woong Kim, Hong Ik Lee,

JiSeong Kim, and Yong K. Kim

Dep‘t of Electronic and Information Engineering, Kunsan National University, Kunsan,

Korea

Thunder Technology Director in Digital Signal Processing Team, ChonJu, Korea

Dep‘t of Information Communication Engineering, Wonkwang University, Iksan Korea

watchbear, [email protected], [email protected]

Abstract. In this study, we designed the OTA circuit for Gm-C active filter

using the direct transform method that has capable of various frequency tuning.

The OTA make it enable filter design that can be used in various frequency

bands due to control trans-conductance by using external voltage control. Our

OTA circuit can be used for active filter of CDMA also other device that use

other frequency bands. From HSPICE simulation, the external voltage from

0.5V to 1.5V, Trans-conductance varies from 2uS to 222uS. The filter is

suitable for any kind of application involving low frequency ranges, and

requiring very low power consumption, such as WCDMA products.

Keywords: Gm-C active filter, OTA circuit, Filter design, Frequency bands

1 Introduction

The OTA is popular for implementing voltage controlled oscillators (VCO) and filters

(VCF) for analog music synthesizers, because it can act as a two-quadrant multiplier

as we’ll see later. For this application the control input has to have a wide dynamic

range of at least 60 dB, while the OTA should behave sensibly when overdriven from

the signal input (in particular, it should not lock up or phase reverse). Viewed from a

slightly different angle an OTA can be used to implement an electrically tunable

resistor that is referenced to ground, with extra circuitry floating resistors are possible

as well.

Most existing work on OTA based filter design approached the problem by either

concentrating upon applying feedback to make the filter characteristics independent of

the trans-conductance gain or modifying existing op amp structures by the inclusion

of some additional passive components and OTAS. In either case, the circuits were

typically component intense and cumbersome to tune. Some of the earlier works are

listed in the Refs. [1-10].

This paper has presented the Gm-C active elliptic filter with variable frequency

band to apply in direct transformation receiver. Since core of filter design is multi

Advanced Science and Technology Letters Vol.122 (ASP 2016), pp.222-228

http://dx.doi.org/10.14257/astl.2016.122.43

ISSN: 2287-1233 ASTL Copyright © 2016 SERSC

mailto:watchbear,%[email protected]

mailto:[email protected]

band characteristics, the filter designs many wireless communication system used as

one equipment or unit, and change and purchase costs are reduced [1].

The direct transform method integrates with Gyrator direct simulation, which

replaced the vast inductor with the trans-conductance (Gm) and the operational Trans-

conductance Amplifier (OTA). The trans-conductance is described as symbol, and

OTA circuits is consisted of main stage to increase the gain of circuit and bias stage to

transform applied voltage into trans-conductance, and CMFB (Common Mode

Feedback) to balance the voltage. The filter is designed the cut-off frequency band

changed as user demands [2]. The analysis of designed filter is simulated with

HSPICE tool for OTA and active filter. The OTA circuit is analyzed for gain

characteristics and trans-conductance deviation according to the change of applied

voltage. The cutoff frequency characteristics and increasing rate of power

consumption is shown for the change of applied voltage according to user demands.

As a results, the gain characteristics of the OTA circuit is decreased with inverse

proportion from 72.3dB to 44.4dB when the applied voltage is changed from 1.0 V to

1.9 V. Trans-conductance characteristics is increased with direct proportion from 2 to

224 when the applied voltage is changed from 0.5 V to 1.5 V. This means that the

active filter consisted of trans-conductance and capacitor is changed. The cutoff

frequency is changed from 1.4MHz to 2.8MHz according to the increasing applied

voltage, and power consumption is increasing from 5mW to 20mW. The target of

design is significantly achieved, and the possibility to be able to apply to the other

frequency bands as well as the WCDMA has been conformed [3-5]

2 OTA Circuit Design

The operational trans-conductance amplifier (OTA) is an amplifier whose differential

input voltage produces an output current. Thus, it is a voltage controlled current

source (VCCS). There is usually an additional input for a current to control the

amplifier's trans-conductance. The OTA is similar to a standard operational amplifier

in that it has a high impedance differential input stage and that it may be used with

negative feedback [1, 6].

The first commercially available integrated circuit units were produced by RCA in

1969 (before being acquired by General Electric), in the form of the CA3080, and

they have been improved since that time. Although most units are constructed with

bipolar transistors, field effect transistor units are also produced. The OTA is not as

useful by itself in the vast majority of standard op-amp functions as the ordinary op-

amp because its output is a current One of its principal uses is in implementing

electronically controlled applications such as variable frequency oscillators and filters

and variable gain amplifier stages which are more difficult to implement with

standard op-amps

For design specification of the OTA circuit, we set unit gain frequency to 15MHz,

phase margin to above 60°, slew rate to 5V/us. The output resistance is set to 10MΩ

and load capacitance is set to 10pF.

Figure 1 shows OTA circuit that has fully differential folded-cascade structure.

Although single input structure, our OTA circuit can achieve high gain using high

Advanced Science and Technology Letters Vol.122 (ASP 2016)

Copyright © 2016 SERSC 223

output resistance due to the characteristics of fully differential folded-cascade

structure [7-9]. Also, our OTA circuit can enhance the CMRR (Common Mode

Rejection Ratio) and the PSRR (Power Supply Rejection Ratio) and reduce noise

dramatically.

Fig. 1. OTA (Operational Trans-conductance Amplifier) Circuit

The block in the M1 ~ M11 of trans-conductance circuit in the figure 2 is differential

input stage. The differential input stage has important parts that using folded cascade

structure and it amplify gain by raising output impedance as possible as it can. The

M12 ~ M22 is bias stage that supply bias voltage to gain & output stage. We can

control frequency band dynamically by varying bias voltage. The M23 ~ M30 is

Common Mode Feedback (CMFB) stage that make voltage to stable if output voltage

level is equal to reference voltage. It gives an advantage that reduces noise level that

occurred in the power supply. It is possible that tunes the trans-conductance by

controlling external bias voltage in bias stage of the OTA circuit. We have designed

the OTA circuit for Gm-C active filter using the direct transform method that has

capable of various frequency tuning. The OTA make it enable to filter design that can

be used in various frequency bands due to control in the trans-conductance by using

external voltage control.

Table 1. Font sizes of headings. Table captions should always be positioned above the tables.

Design parameter Target design specification Simulation result

Unit Gain Frequency >15MHZ 12MHz ~ 16MHz

Phase margin Above 60 90 ~ 90

Slew Rate 5V/ Voltage Gain 40dB 44.4dB ~ 72.3 dB

Output impedance

Load capacitance

Power consumption

10MΩ

10pF

Less than 5mW

-

-

410uW ~1.3mW


224 Copyright © 2016 SERSC

The OTA make it enable filter design that can be used in various frequency bands

due to control the trans-conductance by using external voltage control[3-4]. In another

words, when driving voltage to gate in M12, it can be changed whole bias voltage

(Node8, Node11, Node4), thus varying trans-conductance. By using these

characteristics we can use the OTA circuit to frequency converter[5]. To evaluate

validity of our OTA circuit, we performed computer simulation by using H-SPICE

tools. From simulation result we can adjust trans-conductance from 2uS to 222uS by

driving external bias voltage from 1.0V to 1.9V.

Table 1 shows simulation result and parameters of target design specification. The

design parameters have characteristics with target design specification and simulation

results. It’s less than 15MHz in unit gain frequency within 12MHzthrough 16MHz.

It’s also above 60 within 90~ in phase margin. The Slew rate is 5V/ and 5.88V. We

then make it enable filter design that can be used in various frequency bands due to

control trans-conductance by using external voltage control.

3 Performance Simulation

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Please note that, if your email address is given in your paper, it will also be included

in the meta data of the online version. To evaluate the performance of the proposed

the OTA circuit, we performed simulation by using the H-SPICE tools.

Fig. 2. Gain Characteristic of OTA Circuit



The figure 2 shows gain characteristics of the OTA circuit. The gain has varying from

47dB to 75dB when external bias voltage varying from 1.0V to 1.9V respectively.

And unit frequency band is the 12 ~ 16MHz.

The figure 3 shows phase margin characteristics that has on the 88°~ 93°via bias

voltage varying from the 1.0V to 1.9V. It has satisfies design specification and show

stable performance.

Fig. 3. Phase Margin Characteristics of OTA Circuit

Fig. 4. Characteristics of Trans-conductors According to Bias Voltage.



Figure 4 depicts characteristics of the trans-conductance via bias voltage. When we

drives bias voltage from the 0.5V to 1.5V, trans-conductance varying on 2us to 222us

respectively.

Figure 5 depicts simulation results of slew rates of OTA circuit. Input signal is Unit

Step Function U(t). Result of slew rates is 5.88V/us, it meets requirement of target

slew rates (5V/us) as shown in figure 5.

Fig. 5. Slew Rate Characteristics of OTA Circuit

To increase the slew rate of the circuit, the bias current may simply be increased.

However, in low power applications, the operational amplifier may have a very low

budgeted current. In this case, it is not possible to arbitrarily increase the bias current.

Prior art approaches to increase the load current-to-bias current ratio, and to thereby

increase the slew rate, are stable only over a narrow range of load capacitance.

Conversely, operational trans-conductance amplifiers with dynamic biasing typically

have a fixed the load current-to-bias current ratio and are, therefore, not suitable for

low current applications.

4 Conclusion

A voltage controlled circuits using the OTA as the basic active element have been

presented. The characteristics of these circuits are adjusted with the externally

accessible dc amplifier bias current. Most of these circuits utilize a very small number

of components. Applications include amplifiers, controlled impedances, and filters.

Higher-order continuous-time voltage-controlled filters such as the common

Butterworth, Chebyschev, and Elliptic types can be obtained. In addition to the

voltage control characteristics, the OTA based circuits show promise for high-

frequency applications where conventional op amp based circuits become bandwidth

limited. We designed trans-conductance circuit that can adjust trans-conductance by

controlling external bias voltage. Therefore it can control cut-off frequency of active

filter. To evaluate validity of our OTA circuit, we performed computer simulation by



using H-SPICE tools. From simulation result we can adjust trans-conductance from

2uS to 222uS by driving external bias voltage from 1.0V to 1.9V. Our OTA circuit

can be used for active filter of CDMA also other device that use other frequency

bands.

References

1. Hollman, T., Lindfors, S., Lansirinne, M., Jussila J., Halonen, K. A. I.: A 2.7V CMOS dual-mode

baseband filter for PDC and WCDMA, IEEE Journal of Solid-state Circuits, Vol. 36, pp. 1148-1153, July (2001).

2. Bang, J.-H.: The Design of A CMOS Gm-C Lowpass Filter with Variable

3. Lee, W., Bang, J.: A Multi-channel CMOS Low-voltage Filter with Newly Current-mode Integrator. 4. Kim, S. H.: Folded-Cascode Operational Amplifier for 32X32 IRFPA Readout Integrated Circuit

using the 0.35μm CMOS process. School of Nano engineering, Inje University 5. Razavi, B.: Design of Analog CMOS Integrated Circuit. pp 122-125, pp 139-146

6. Choi, H.-J., Choi, J.-K., Kim, H.-S.: A Design of Low Frequency Noise Figure

Improvement of RF Circuit for Direct Conversion Receiver. ITFE J. of Summer

Conference, Vol. 2009, pp. 305-308, August. (2009). 7. Bang, J.-H.: “The Design of A CMOS GM-C Lowpass Filter with Variable Cutoff Frequency for

Direct Conversion Receiver” The Transactions of the Korean Institute of Electrical Engineers. Vol.57,

pp.1464-1469, Aug (2008). 8. Lim, S.H.: A channel-selection switched capacitor filter. Proc. IEEE 47th Midwest Symp. Circuits

and Systems, pp. I-117-I-120, Hiroshima, Japan, July (2004).

9. Jeong, T., Bang, J.: CMOS Low-voltage Filter For RFID Reader Using A Self-biased Transconductor. KAIS. Vol.10, pp. 1526-1531, July. (2009).

10. Wittlinger, H.A.: Applications of the CA3080 and CA3080A High Performance Operational Transconductance Amplifiers. RCA Application Note ICAN-6668.



Implementation of the OTA Circuit Design for Gm-C Active Filter

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