cmos transconductance amplifier types for low power

XXIII Congresso Brasileiro em Engenharia Biomédica – XXIII CBEB

CMOS TRANSCONDUCTANCE AMPLIFIER TYPES FOR LOW POWER ELECTRICAL IMPEDANCE SPECTROSCOPY M. M. Santos1, P. Bertemes-Filho1 e V. C. Vincence1 1

Department of Electrical Engineering/State University of Santa Catarina, Joinville, Brazil [email protected]

applications was reported in [9,14], which is based on the operational transconductance amplifier (OTA) approach. In particular, the design in [9] operates in weak inversion, its operating frequency is limited to 20 kHz and its current drive is limited to a few microamperes. Our targeted measuring applications require an operating frequency band of 1 kHz to 1 MHz and current amplitudes of up to 500 µAp (peak). A VCCS for EIS may be analog or mixed-signal. In addition, it can be single-ended or fully-differential and the system may include a single or multiple current sources [10]. Single-ended systems always have large common-mode voltages since one end is tied to virtual ground [11]. Although transformer coupling can reduce this effect [11], the use of a transformer is not suitable for an integrated solution. Reducing the common-mode voltage without decreasing the output impedance of the source is required. Therefore, a floating current source is desirable because it decreases the common-mode voltage and increases the output impedance. Two OTA-based floating VCCSs for wideband EIS instrumentation are described in the paper and are compared to the second generation of a Current Conveyor circuit. The circuits were designed in a 0.35 µm CMOS technology and simulation results are presented.

Abstract: Electrical Bioimpedance Analysis have been widely used as a non-invasive technique for characterizing tissues. Most systems use a wideband and a high precision instrumentation, specially the current source. The objective of this work is to compare the Current Conveyor circuit with two OTA-based floating voltage controlled current sources. The results show that the class-A OTA has a wider output current frequency response. On the other hand, the output impedance of both Class-AB OTA and Current Conveyor is 4 times higher than the Class-A OTA at 1 MHz. In terms of power consumption, the Class-A OTA circuit consumes almost 3 times more power than both Class-AB OTA and Current Conveyor circuits. The results might be useful for cell impedance measurements and for very low power bioimpedance applications. Keywords—CMOS Current Source, Transconductance Amplifier, Electrical Bioimpedance, Low Power. Introduction Electrical Bioimpedance Analysis (also called Electrical Impedance Spectroscopy - EIS) has been widely used as a non-invasive technique for detecting cancerous tissues. Also, it is considered fast, of low cost, practical and efficient [1]. This technique has been shown good results for detecting normal and cancerous skin superficial tissues [2,3]. Most EIS systems consist of applying a multi-frequency sinusoidal current of constant amplitude in the tissue sample, measuring the resulting potential and then calculating the transfer impedance (Zt) [1]. In order to get an accurate calculated transfer impedance lower than 1%, it is necessary to assure that the injecting current has a constant amplitude over a wide frequency range, which may be obtained by using a current source with high output impedance [1]. However, stray capacitances reduce the current amplitude at higher frequencies [4]. High performance current source with a wide bandwidth by using operational amplifiers has been widely developed and found in the literature [5,6,7] and others use integration techniques for developing current source [8,9,10,11,12,13]. An integrated realization of a voltagecontrolled current source (VCCS) for bioimpedance

Materials and Method Both Current Conveyor circuit and the two types of CMOS class-A and AB Transconductance Amplifier were implemented in Orcad PSPICE simulator (2009 version). The last current source was obtained by a modification in its output stage. Simulations were performed in order to obtain linearity, output current and impedance response in the frequency range from 10 Hz to 1 GHz. Current Conveyor – The current conveyor can be implemented by using translinear structures [15,16,17] or differential pair [18,19]. Figure 1 shows a current conveyor circuit based on two differential pairs of transistors M1-M2 and M3-M4, which allows a rail to rail input stage [20].

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Fig. 1 Diagram schematic of the simulated Current Conveyor.

The transistors M13 and M14 are source followers which are used to keep a low resistance in the X terminal and to stabilize the bias current throw the transistors M8 e M12. The circuit showed in figure 1 is modified from [18]. It is used low voltage current mirrors and a differential output stage. The current flowing in the M13-M14 are copied to the output nodes Z1 and Z2. The phase of the output current at Z2 node is 180° phase shifted compared to the Z1 node. The differential output current (Io) is calculated by the relationship VY/R1 (see figure 1). The circuit linearity (Io versus VY) was investigated by varying the sinewave magnitude of VY from -0.6 to +0.6 Volts. The output current frequency response was measured directly across a 1 kΩ load. The circuit is supplied by a voltage source of ±1.5 Volts. Class-A OTA – This is an Operational Transconductance Amplifier (OTA) implemented to operate as a class-A amplifier. It uses a semiconductor based on CMOS structural length down to 0.35 µm, which is manufactured by Austria Microsystems. The circuit was implemented to have a common mode control through the transistors M6 and M7 operating in the triode region (see figure 2). It has a cascode output structure by using the transistors M8 to M13 and source degeneration by using MOS transistors MA and MB operating in the triode region. The current source was projected to have a transconductance of 1 mS (see equation 1) and an output current of 1 mApp (peak to peak) over the frequency range 1 Hz to 1 MHz. The circuit is supplied by a voltage source of ±1.5V (VDD= -VSS), where Ib is a external bias current set to 500 µA, V1,2 are voltage input signals, Vcas1,2 and Vbias are dc voltages of the cascode pairs and Vout+ and Vout- can

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be considered as a fully-differential output, where the currents Iout- and Iout+ will flow through the load. 2/ 1/

(1)

where gm1 is the transconductance of transistor M1, gds is the conductance of transistor MA. The output resistance is approximately equal to the product of gm9, ro9 and ro11, where ro11 is the output resistance of transistor M11 and ro9 and gm9 are the output resistance and conductance of the transistor M9, respectively.

Fig. 2 Diagram schematic of the simulated CMOS class-A operational transconductance amplifier (modified from [13]).

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Class-AB OTA – This is an Operational Transconductance Amplifier (OTA) implemented to operate as a class-AB amplifier. It uses the same technology as class-A OTA described before. Basically, it contains a main functional structure (see M1, M2, M5,

Ma and Mcas transistors in figure 3), which is a modified structure proposed by Carvajal [14] called flipped voltage follower.

Fig. 3 Diagram schematic of the simulated CMOS class-AB operational transconductance amplifier (modified from [14]).

This is a cell of low voltage and low power developed to work as a class-AB circuit, where the Ma transistor (see figure 3) is polarized by the constant current source Ib so that the voltage applied in the gate of this transistor is copied to its source as a positive displacement voltage Vgs. The transistors M1 and M2 are connected in low impedance nodes, which are supplied by drains of the transistors M5 and M6, respectively. The voltages Va and Vb are projected so that the differential transistor pairs M1,2 and M3,4 work as a fully differential class-AB voltage input. The output stage formed by the transistors M7, M9, M11, M13, M15, M17, M19, M21, M23 and M25 makes the subtraction of the currents “i2+i4” (i2 is the current through transistor M2, for example) and “i1+i3”, resulting in a linear structure. From the other output stage, at Vout+, the output current is given by (i1+i3)((i2+i4). As a result, the total output current which flows through the load is fully differential.

be calculated that the power consumption of both classAB and Current Conveyor is 1.6 mW whereas 4.6 mW for class-A OTA. Figure 5 shows the frequency response of the current sources. It can be observed that the class-A OTA can drive 1 mApp across 1 kΩ load in a frequency range from 10 Hz to approximately 20 MHz. It can be calculated from figure 5 that the corner frequency in this type of current source is approximately 567 MHz. It can also be observed that the corner frequencies of the classAB OTA and Current Conveyor circuits are approximately 37 and 33 MHz, respectively.

Results It can be seen in figure 4 that the Class-AB OTA current source is very linear presenting a constant ratio of 1 mS from -0.6 to +0.6 Volts. On the other hand, the output current of the class-A OTA is not linear over the input voltage range -0.6 to +0.6 V, but it is linear between an input voltage from -0.4 to +0.4 V. The linear bandwidth of the class-A structure can be increased by optimizing the cascode pairs at the output stage [15]. Also it can be observed in figure 4 that the Current Conveyor is more linear than the class-A with an input voltage range -0.5 to +0.5 V. From figures 1 and 3 it can

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Fig. 4 Simulated linearity response of the current source over the input voltage range -0.6 to +0.6 V, using a resistive load of 1 kΩ. It can be observed in figure 6 that the output impedance frequency response of both Current Conveyor and Class-AB OTA circuits is identical. They have a maximum output impedance of approximately 157.4 dB whereas 131.5 dB for the class-A OTA. However, it decreases to approximately 129 dB at 1


MHz for Current Conveyor and Class-AB OTA circuits whereas 116.8 dB for the Class-A OTA circuit.

Biomed. Eng., CIDADE, PAÍS, 1998, pp 2886-2889. [3] González-Correa, C A, Brown, B H, Smallwood, R H et al (1999) Virtual Biopsies in Barrett's Esophagus using an Impedance Probe. Annals New York Academy of Sciences 873:313-321. [4] Lu, L, Brown, B H (1994) The electronic and electronic interface in an EIT spectroscopy system. Inn. Tech. Biol. Med. 15:97-103. [5] Bertemes-Filho, P, Brown, B H, Wilson, A J (2000) A comparison of modified Howland circuits as current generators with current mirror type circuits, Physiol. Meas. vol 20, pp.1-6. [6] Bertemes Filho, P, Lima, R G, Amato, M B P et al (2006) "Performance of an Adaptative Multiplexed Current Source used in Electrical Impedance Tomography”. XX Brazilian Congress Biomed. Eng., Sao Pedro, Brazil, 2006, pp. 1167-1170.

Fig. 5 Simulated output current magnitude over the frequency range 10 Hz to 1 GHz.

[7] Seoae, F, Bragós, R, Lindecranz, K (2006) “Current source for multifrequency broadband electrical bioimpedance spectroscopy systems: a novel approach”, IEEE Proc. of the EMBS Annual Int. Conf., New York, USA, 2006, pp 5121-5125. [8] Yufera, A, Rueda, A, Munoz, J M et al (2005) “A tissue impedance measurement chip for myocardial ischemia detection”, IEEE Transactions Circuits Syst., vol 52 no 12. [9] Tsunami, D, McNames, J, Colbert, A et al (2004) “Variable Frequency Bioimpedance Instrumentation”, IEEE Proc., Annual Int. Conf. of the IEEE EMBS, San Francisco, USA, 2044, 1-5. [10] Hong, H, Rahal, M, Demosthenous, A et al (2007), “Floating Voltage-Controlled Current Sources for Electrical Impedance Tomography”, 18th European Conference on Circuit Theory and Design, 2007, pp 208-211 DOI 10.1109/ECCTD.2007.4529573.

Fig. 6 Simulated output impedance over the frequency range 1 Hz to 10 MHz.

[11] Uranga, A, Sacristán, J, Osés, T et al (2007) Electrode-tissue Impedance Measurement CMOS ASIC for Funtional Electrical Stimultion Neuroprostheses, IEEE Transactions on Inst.&Meas. vol. 56 n. 5, pp 2043-2050. [12] Carvajal, R G, Ramírez-Angulo, J, López-Martín, A et al (2005) The Flipped Voltage Follower: A Useful Cell for Low-Voltage Low-Power Circuit Design, IEEE Transactions on Circuits and Systems vol. 52 n. 7 pp 1276-1291.

Discussions and Conclusion The output current and output impedance of the Current Conveyor current source were compared to a both modified class-A and class-AB OTAs. It was showed that the class-AB OTA has a better performance in the frequency range 10 Hz to 1 MHz than the others current source simulated in this article. These results are encouraging the authors to obtain a practical integration, where the advantages of the miniaturization and low power might be interesting for micro-implants and cell impedance measurements. Acknowledgment I thank the State University of Santa Catarina (UDESC) for the financial support. References

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