Design of High gain and Low Offset CMOS Current Mode Front End Operational Amplifier

Transcription

1 Design of High gain and Low Offset CMOS Current Mode Front End Operational Amplifier R.SHANTHA SELVA KUMARI 1, M.VIJAYALAKSHMI 2 1 Professor and Head, 2 Student, Department of Electronics and Communication Engineering, Mepco Schlenk Engineering College, Tamil Nadu, India. viji.lakshmim@gmail.com Abstract: - The trends toward low voltage low power silicon chip system have been growing due to the increasing demand of smaller size and long battery life for biomedical applications. This paper deals with welldefined design criteria for CMOS current mode operational amplifiers. A simple design procedure is presented which has current mode preamplifier with an active feedback loop has operated at low frequency to bypass the DC offset current. Unlike previous methods, this approach yields high gain performance without introducing unnecessary circuit constraints. This paper presents the operational amplifier schematic implementation and simulation results using Advance Design System tool. The characteristics are verified by using 0.18µm CMOS technology. The required power supply voltage is reduced to 1.8V from 3V. Key-Words: - Current mode, preamplifier, front end, Active feedback loop, low DC offset, TSMC 0.18µm 1 Introduction As CMOS Technology shrinks towards Nano scale regime, there is an increasing demand for more advanced and effective medical devices in biomedical applications like neural recording system. These systems usually contain various types of biosensors. These biosensors have CMOS operational amplifier as one of the important block at the front-end of the system to sense and amplify the signal. For the neural recording system, there will be increase in the number of input electrodes required for clinical and implant applications in which front end system must meet the requirements of low power consumption. They have to provide amplification selective to the physiological signal, reject superimposed noise and interference signals and guarantee protection from damages through voltage and current surges for electronic equipment. The main task of a neural signal preconditioning module is the amplification of weak neural signal as well as filtering out the frequencies beyond the bandwidth of interest. This is done by using preamplifier combined with filter circuit. The most critical block in neural recording system is low power low noise front end neural amplifier which is the first stage. The neural signal which is received from electrode tissue interface is amplified by the front end amplifier with less distortion effect. In some cases, both gain and cut off frequencies of signal preconditioning module are at least tunable. The AC coupled techniques are designed in some of the voltage mode front end amplifier to reject the DC offset voltage. MOS pseudo resistor is adopted to realize ultra large resistance which are highly dependent on voltage across them. This approach [3] requires reset signal to reset the pseudo resistor and keep the bias condition stable. The use of pseudo resistor leads to large signal distortions and makes the circuit design more complicated. Reset signal is required for resetting the pseudo resistor which in turn keeps the bias condition stable. This results in complex design of the circuits. The use of discrete components [1] like external capacitor increases the size of the acquisition device thereby limiting the use of the device in implantable application.there are several current mode circuits used for amplification of the signal. Several current-mode circuits [3] have been proposed for amplifier applications. In [6], a CMOS front-end amplifier for photo-current acquisition is presented with a feedback loop consisted of a sample-and-hold stage, an error amplifier, and a sinking device to bypass the flowing-out dc current. Current mode circuit is designed to have low impedance characteristics which retain the advantages of low power consumption and low power supply. It also has a good noise rejection capability. As compared to the voltage mode amplifiers, ultra high value pseudo resistor with reset signal and external capacitor are not required. Current mode preamplifier with the cancellation feedback loop is designed to bypass the dc offset current generated ISBN:

2 by the electrode-tissue interface so that the dc offset current does not affect the amplifier operation. Due to the existence of high-impedance nodes, the drawbacks of voltage-mode circuits, such as limited bandwidth and the need for a high supply voltage, cannot be avoided. In addition, this configuration requires a current-to-voltage conversion stage, usually a passive resistor, which deteriorates the noise performance. current both in common source, common gate amplifier configurations and common drain amplifier configuration are insignificant at low supply voltages because of the bulk-effect present in typical CMOS-processes. 2 Design of Current Mode Front End Amplifier The front end amplifier is an electronic device that serves to boost low power signal to higher power signal that is usable to work. There are two reasons to amplify the signal. First, Amplification increases the level of the signal to protect from electrical interference during transmission. Second, the signal is amplified so that it could be stored in a storage device, or displayed by measurement device like oscilloscope. Recently various topologies have been explored to get low noise and high power efficiency. The one stage operational with current mirror is among the Operational Transconductance Amplifier (OTA) technique for neural amplifier design. This OTA topology can achieve wide output but relatively low gain. Cascode transistors are added to the output branch to enhance the gain at the cost of reduced output. Since the current mirror approach contributes noise and consumes power Operational Transconductance topology is not efficient. Low noise amplifier with filtering capability is needed for the front end system to amplify the desired signals and eliminate direct current baseline. The size and power consumption need to be minimized to reduce the heat dissipation which results in tissue damage for human applications. Hence low noise front end amplifier is required to reject the noise. The block diagram of current mode front end amplifier is shown in fig.1 which consists of current-mode preamplifier with an active feedback loop. The current mode preamplifier is the first stage of the current mode amplifier to amplify the signal and the second stage is the active feedback loop to bypass the dc offset current generated from the electrode-tissue interface. Current mode circuit realisation is to use current signal rather than voltage signal for signal processing. MOS transistors are more suitable for processing currents rather than voltages because the output signal is Fig.1. Block diagram of Current mode front end amplifier CMOS current mirrors are more accurate and less sensitive to process variation than bipolar current mirrors because with the latter the base current limits the accuracy. Therefore, MOS transistor circuits should be simplified by using current signals in preference to voltage signals. For this reason, integrated current mode realisations are closer to the transistor level than the conventional voltage-mode realisations and therefore these results in simpler circuits and systems 2.1 Design of Current Mode Preamplifier The preamplifier represents the most critical part of the amplifier itself since it contributes the quality of the signal. The main tasks of the preamplifier are to sense the signal and minimizing the effect of electrode potentials. The structure of the current mode preamplifier with low DC current is shown in fig.2 (a).the operational amplifier OP 1 is designed with two stage topology as shown in fig. 2(b).The first block of two stage amplifier is a differential amplifier. It has two inputs which are the inverting and non-inverting voltage. It provides at the output a differential voltage or a differential current that depends on the differential to single ended converter. The next stage is a differential to single ended converter. It is used to transform the differential signal generated by the first stage into single ended conversion. The gain is provided by intermediate stage is not sufficient for the input stage and additional ISBN:

3 amplification is required. This is provided by the intermediate stage which is another differential amplifier driven by the output of the first stage. As this stage uses differential input unbalanced output differential amplifier, so it provide required extra gain transistor M 6 does not provide biasing for M 5, indeed M 6 is biased from the gate side. (a) in the linear subthershold region as resistor. The channel lengths of M p1 (M n1 ) and M p2 (M n2 ) are same. The resistance ratio of the channel is equal to the channel width which is designed to be 40. This ratio provides current gain of 40. The gain of this system is also very important. The signals must be amplified to a reasonable level for the detection of the neural signals. The neural signals can be as low as micro volts, but they can get higher for situations when there is a very high sealing resistance. The gain should be achieved as maximum as 56 db and to achieve optimum noise performance. The equivalent electrode-tissue interface with the current mode amplifier is shown in fig.3. R s is the spreading resistance between electrolyte and tissue, R e and C e are the resistance and capacitance of the electrode-tissue interface, R m is the resistance of the electrode and R in is the input impedance of Current mode front end amplifier. The input impedance of the current mode front end amplifier should be designed as low as possible which leads to less degradation effect on the system. Fig.3 Equivalent circuit of electrode tissue interface (b) Fig.2 (a) Structure of current-mode preamplifier. (b) Circuit schematic of Op 1 The input resistance of the Current mode preamplifier is measured by the given equation The second stage consists of M 6 which is common source amplifier actively loaded with the transistor M 5. The transistor M 6 does not provide biasing for M 5, indeed M 6 is biased from the gate side. The smallest device that will keep the channel modulation parameter constant and provide good matching for current mirror which has been chosen. The size of M 1 M 4 is enlarged to decrease the offset caused by the device mismatch. The devices in Figure.2 (a) are operated at a very low DC current R in is determined by the resistance of R MN1 and R MP1 and A op1 gain of the operational amplifier OP 1 which is two stage operational amplifier. R MN1 and R Mp1 are the turn on resistance of the M n1 and M p1. To avoid the effect of long length in limiting the signal swing M 3 is designed with large width since the input noise is independent of the width W 3. The width of the transistor W 1 is also designed with large width to reduce the noise. ISBN:

4 2.2 Design of Active feedback Loop The active feedback loop operation differs from that of traditional Opamp. It provides beneficial separation between the signal input and the feedback network. The feedback loop has high output resistance with miller compensation capacitor to achieve the low cut off frequency. Feedback amplifier is usually used in the design of amplifiers in order to benefit from some of the following advantages: Desensitization of the closed-loop gain against variations of open-loop circuit parameters Improvement of the impedance seen into the input and output ports of the amplifier Fig.4 (a) Structure of Feedback loop. (b) Schematic circuit of Telescopic Opamp The input dc offset current is determined by the resistance of the electrode-tissue interface and the input resistance of the preamplifier as well as the DC offset voltage across of the electrode-tissue interface and the input dc offset voltage of the preamplifier. Thus both the offset voltages should be kept as small as possible to minimize the input dc offset current. The total input dc offset voltage can be minimized to tens of micro volts. The total DC resistance of the preamplifier input loop is in the mega ohm order. Thus, the input dc offset current can be minimized to avoid the damage to the tissue or electrodes. When the input dc offset current flows into the preamplifier, the output voltage of is changed to drive which generates a similar dc current to bypass the input dc offset current. The loop follows the telescopic architecture with common source stage as as shown in Figure. 4(b). The telescopic operational amplifier offers the least power consumption, lowest noises, and highest speed. The telescopic architecture puts both input differential pair and the output on the same current branches which eliminates the noise problem caused by the current mirrors. This type of amplifier is a better candidate for low power, low noise single stage operational amplifier. The output noise of the OP2 in Fig. 4(a) contributes the noise current to the input node through Mx transistor. It is designed to have to be at zero operating current, the generated noise current is very small. Thus, the noise contribution of operational amplifier (OP2) and Mx transistor can be neglected. The amount of the bypass current can be derived as Where R M1 is the resistance of Mn1 and M p1, g mx is the transconductance of M x, A fed is the gain of Op 2 and I os (a) 3 Results & Discussion The two stage amplifier Figure. 2 (b) is designed and it is simulating using ADS and the gain of the amplifier is shown in fig.7. ISBN:

5 Fig 5. Frequency responset of Two stage Opamp The two stage operational amplifier act like booster to provide enough gain of 60 db. When using smaller channel lengths, the width of the input transistors is increased and high gain from the input to the output is maintained. The Current mode front end amplifier Figure. 4 (a) is designed and it is simulating using ADS and the gain of the amplifier is shown in Figure7. \When the input dc offset current flows into the preamplifier, the output of the operational amplifier is changed to drive the dc current to bypass the DC offset current which is measured as 6.28µA. Figure 6. Frequency response of Current mode amplifier 4 Conclusion There are several approaches which have been proposed for amplifier application to sense and amplify the neural signals. The current mode amplifier is a good choice as low voltage amplifier for current deep submicron CMOS technology that provides optimised results suitable for neural recording application. The approach of current mode circuit is designed with high gain factor and less distortion effect. The amplifier rejects DC offsets commonly encountered in microelectrode recording applications. The DC offset current flows into the feedback path and is reduced without affecting the in-band signal. The required minimum supply voltage is reduced to 1.8V. The current mode circuit is designed with preamplifier and active feedback loop in TSMC 0.18µm CMOS technology. The current mode circuit has been simulated using Advanced Design System. The future work is to design the current mode circuit with programmable gain stage to adjust the gain factor for different signal to achieve the compatibility of the system. References: [1] R.R Harrison and C.Charles, A low-power low-noise CMOS amplifier for neural recording applications, IEEE J. Solid-State Circuits, vol. 38, no. 6, pp , Jun [2] R. R. Harrison, A versatile integrated circuit for the acquisition of biopotentials, in Proc. IEEE Custom Integrated Circuits Conf., Sep.2007, pp [3] A.K.Y. Wong, K.P.Pun,Y.T.Zhang, and K.N.Leung, A low power CMOS front end for photoplethysmographic signal acquisition with robust DC photocurrent rejection, IEEE Trans.Biomed.CircuitsSyst.,vol.2,no.4,PP ,Dec [4] M.Yin and M.Ghovanloo, A low-noise preamplifier with adjustable gain and bandwidth for biopotential recording applications, in Proc.IEEE Int. Symp. Circuits Syst., May 2007, pp [5] G.Ferrari, F.Gozzini, A.Molari, andm. Sampietro, Transimpedance amplifier for high sensitivity current measurements on nanodevices, IEEE J. Solid-State Circuits, vol. 44, no. 5, pp , May [6] Kaulberg.T, A CMOS Current mode operational amplifier IEEEJournal 1993,Pages: [7] G.Ferrari, M.Farina, F. Guagliardo, M. Carminati, and M. Sampietro, Ultra-low-noise CMOS current preamplifier from DC to 1MHz, Electron. Lett., vol. 45, no. 25, pp , Dec. 3, [8] M. Yavari and O. Shoaei Low-voltage lowpower fast-settling CMOS operational transconductance amplifiers for switchedcapacitor applications IEEE Proc.-Circuits Devices Syst., Vol. 151, No. 6, December ISBN:

6 [9] N. Verma, A. Shoeb, J. Bohorquez, J. Dawson, J. Guttag, and A. P.Chandrakasan, A micropower EEG acquisition SoC with integrated feature extraction processor for a chronic seizure detection system, IEEE J. Solid-State Circuits, vol. 45, no. 4, pp , Apr [10]. [10] Wang Yufeng,Wang Zhigong.Lu Xiaoying,et al. A single chip and low power CMOS amplifier for neural signal detection Chinese Journal of Semiconductors,2006,27:1490(in Chinese). [11] R. Martins, S. Selberherr, and F. A. Vaz, A CMOS IC for portable EEG acquisition systems, IEEE Trans. Instrum. Meas., vol. 47, pp ,1998. [12] K. Gulati and H.-S. Lee, A high-swing CMOS telescopic operational amplifier, IEEE J.Solid- State Circuits, vol. 33, pp , Dec [13] Priyanka Kakoty, Design of a high frequency low voltage CMOS Operational amplifier, International Journal of VLSI design & communication System (VLSICS), Vol.2, No.1, pp , March ISBN: