Advanced Science and Technology Letters Vol.122 (ASP 2016), pp.222-228 http://dx.doi.org/10.14257/astl.2016.122.43
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,
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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
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Advanced Science and Technology Letters Vol.122 (ASP 2016)
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 Transconductance 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 opamp 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
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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 Unit Gain Frequency Phase margin Slew Rate Voltage Gain Output impedance Load capacitance Power consumption
224
Target design specification >15MHZ Above 60 5V/ 40dB 10MΩ 10pF Less than 5mW
Simulation result 12MHz ~ 16MHz 90
~ 90
44.4dB ~ 72.3 dB 410uW ~1.3mW
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Advanced Science and Technology Letters Vol.122 (ASP 2016)
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
The online version of the volume will be available in LNCS Online. Members of institutes subscribing to the Lecture Notes in Computer Science series have access to all the pdfs of all the online publications. Non-subscribers can only read as far as the abstracts. If they try to go beyond this point, they are automatically asked, whether they would like to order the pdf, and are given instructions as to how to do so. 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
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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.
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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 highfrequency 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
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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.
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