Regular paperA rail-to-rail constant-gm CCII for Instrumentation Amplifier applications
Introduction
The continuous and ever-increasing demand of portable electronic devices with prolonged autonomy such as, for instance, in electromedical, wireless sensor network and Internet of Things applications, requires the development of challenging circuit solutions and design techniques aimed at further minimizing power consumption maintaining good general performances [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. In this perspective, Instrumentation Amplifiers (IAs) are widely used in data acquisition systems and signal processing applications. Particular kinds of IAs, based on the current-mode approach, are the Current-Mode Instrumentation Amplifiers (CMIAs) [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. The most important performance parameter of an IA is the Common Mode Rejection Ratio (CMRR) which indicates how well it is possible to measure the desired differential signal under the presence of large unwanted common mode ones. Generally, the CMRR of an IA not only depends on the matching between the used active elements but also on the external resistor tolerances. Moreover, the supply voltage also greatly affects IA performances; in particular, decreasing the supply voltage reduces the maximum number of stacked gate-source voltages and saturation voltages allowed between the supply rails. Signal swing may be also severely impaired and, to mitigate this drawback, complete dynamic (rail-to-rail) topologies must be selected. Specifically, as far as input differential stages is concerned, rail-to-rail solutions are implemented through two complementary source-coupled pairs connected in parallel. This requires also the control of the total transconductance (gm), which has to be kept constant over the whole input common-mode voltage range, so that power-efficient frequency compensation techniques providing a constant Gain-Bandwidth (GBW) can be employed [25], [26], [27], [28], [29], [30], [31], [32].
All these considerations can be directly applied to the implementation of a low-voltage current mode IA. This work presents the design of a constant-gm rail-to-rail low voltage CCII-based IA operating at a 1.5 V power supply showing a 177 μW power consumption and a CMRR of about 105 dB.
Section snippets
The proposed constant-Gm rail-to-rail CCII
In order to develop a constant-gm CCII, the simplified block scheme in Fig. 1 is adopted. It consists of three stages. The first one (input stage) is an Operational Transconductance Amplifier (OTA), made up of two complementary input pairs in parallel providing rail-to-rail operation (plus a control circuitry ensuring a constant-gm operation for the input stage, not shown). The output currents of the two complementary pairs are added at their single-ended output. The second stage (INV X) is an
The proposed CCII based rail-to-rail IA: simulation results
The basic block scheme of the proposed current mode IA is shown in Fig. 4 [23], [24]. It is based on two CCIIs as active building blocks which transfer Y terminal voltage to X one and converts it to a current using a resistor. The current produced at X terminal is copied to output (Z terminal). The ideal input-output relationship is given in Eq. (10).
Simulation results
The complete circuit was designed in a standard CMOS technology (0.35-µm by AMS) designing two identical CCII. Transistor dimensions of the single CCII are shown in Table 1. For the single CCII, at a 1.5 V supply voltage, DC power dissipation is 88.5 µW. Always concerning the standalone CCII, Fig. 5 shows the transconductance of both the n- and p-channel CCII differential pairs, as well as the total transconductance gmT, given by the sum of the two gm’s, that can be considered constant with the
Conclusions
We have proposed a current mode Instrumentation Amplifier (IA) based on second generation current conveyor (CCII) with rail-to-rail and constant-gm characteristics. Simulation results have shown the validity of the proposed architecture for integrated circuits for low-voltage portable applications.
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2021, AEU - International Journal of Electronics and CommunicationsCitation Excerpt :It consists of a constant-gm control circuit, two complementary input pairs providing rail-to-rail operation, a class AB output stage and the frequency compensation circuit. The folded cascode operational transconductance amplifier (FC-OTA) included a nMOS and a pMOS differential pairs is often adopted to achieve the rail-to-rail input of an amplifier such as the techniques proposed in [11–15]. However, owing to the increased current path formed by the additional input pairs, the traditional FC-RtR amplifiers usually consume more power than other general ones.
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