J.M. Carrillo

1-V Rail-to-Rail CMOS OpAmp With Improved Bulk-Driven Input Stage

This paper introduces a CMOS operational amplifier with rail-to-rail input and output voltage ranges, suitable for operation in extremely low-voltage environments. The approach is based on a bulk-driven input stage with extended input common-mode voltage range, in which the effective input transconductance is enhanced by means of a partial positive feedback loop. As a result, a gain and gain-bandwidth product performance similar to that of an amplifier using a gate-driven approach is obtained. Output rail-to-rail operation is achieved by means of a push-pull stage, which is biased in class-AB by using a static feedback loop, thus avoiding frequency limitations inherent in dynamic-feedback tuning schemes. The proposed two-stage operational amplifier was designed to operate with a 1-V supply, and a test chip prototype was fabricated in 0.35-mum standard CMOS technology. The experimental performance features an open-loop DC gain higher than 76 dB and a closed-loop unity-gain bandwidth above 8 MHz when a 1-MOmegapar17-pF load is connected to the amplifier output.

Constant-g/sub m/ constant-slew-rate high-bandwidth low-voltage rail-to-rail CMOS input stage for VLSI cell libraries

This paper introduces a general-purpose low-voltage rail-to-rail input stage suitable for analog and mixed-signal applications. The proposed circuit provides, simultaneously, constant small-signal and large-signal behaviors over the entire input common-mode voltage range, while imposing no appreciable constraint for high-frequency operation. In addition, the accuracy of the circuit does not rely on any strict matching of the devices, unlike most of the traditional approaches based on complementary input pairs, which need to compensate for the difference in mobility between electrons and holes with the transistor aspect ratios. Also, the technique is compatible with deep submicrometer CMOS devices, where the familiar voltage-to-current square law in saturation is not completely satisfied. Based on the proposed input stage, a transconductor with rail-to-rail input common-mode range and an input/output rail-to-rail operational amplifier were developed. Both cells were designed to operate with a 3-V single supply and fabricated in standard 0.8-/spl mu/m CMOS technology. Experimental results are provided.

On the input common-mode voltage range of CMOS bulk-driven input stages

In this paper the response of a bulk-driven MOS Metal-Oxide-Semiconductor input stage over the input common-mode voltage range is discussed and experimentally evaluated. In particular, the behavior of the effective input transconductance and the input current is studied for different gate bias voltages of the input transistors. A comparison between simulated and measured results, in standard 0.35-µm CMOS Complementary Metal-Oxide-Semiconductor technology, demonstrates that the model of the MOS transistors is not sufficiently accurate for devices operating under forward bias conditions of their source-bulk pn junction. Therefore, the fabrication and the experimental evaluation of any solution based on this approach are highly recommended. A technique to automatically control the gate bias voltage of a bulk-driven differential pair is proposed to optimize the design tradeoff between the effective input transconductance and the input current. The proposed input stage was integrated as a standalone block and was also included in a 1.5-V second-order operational transconductance amplifier (OTA)-C lowpass filter. Experimental results validate the effectiveness of the approach. Copyright

A Family of Low-Voltage Bulk-Driven CMOS Continuous-Time CMFB Circuits

This brief introduces four different structures for implementing a continuous-time common-mode feedback (CMFB) network for fully differential (FD) amplifiers. The proposed circuits use bulk-driven MOS transistors, thus representing a low-voltage realization of their gate-driven counterparts. The CMFB circuits were included in a 1.5-V FD buffer implemented in standard 0.35-μm CMOS technology. Experimental results illustrate the performance of the proposed schemes, demonstrating their suitability to operate with a low supply voltage.

1-V continuously tunable CMOS bulk-driven transconductor for G m -C filters

A bulk-driven transconductor including a continuous tuning scheme and operated from a 1-V supply is presented in this paper. The voltage-to-current conversion is carried out by means of the source degeneration of a bulk-driven differential pair, while transconductance tuning is obtained by modifying the gain of a current mirror. The proposed circuit has been included in a continuous-time transconductance-C (Gm-C) biquadratic cell designed for the audio bandwidth, which features a programmability range of more than one decade and a dynamic range of 63.7 dB while consuming less than 45 muW.

1-V rail-to-rail bulk-driven CMOS OTA with enhanced gain and gain-bandwidth product

This paper introduces a CMOS amplifier input stage with extended input common-mode voltage range, suitable for operation in extremely low-voltage environments. The approach is based on a bulk-driven input differential pair, in which the gain and the gain-bandwidth product are enhanced by means of a partial positive feedback loop. The bulk-driven input transistors allow obtaining a very wide input voltage range, while the partial positive feedback leads to an effective transconductance improvement. The proposed input stage has been used to design a 1-V OTA. Post-layout simulated results, obtained in 0.35-/spl mu/m standard CMOS technology, are given.

Feedback vs. feedforward common-mode control: a comparative study

A comparison among feedforward (CMFF) and the traditional common-mode feedback (CMFB) loops, based on the most frequently used common-mode (CM) signal detectors for CM control in fully-differential (FD) circuits, is carried out. Simulated results confirm that CMFF shows a better performance in terms of induced nonlinear signal distortion, speed, and amplifier output signal swing. It is demonstrated that feedforward approach results very attractive for low-voltage applications.

Input/Output Rail-to-Rail CMOS Operational Amplifier with Shaped Common-Mode Response

This paper presents an input/output rail-to-rail class-AB CMOS operational amplifier with reduced variations in unity-gain frequency over the entire voltage range. The rail-to-rail amplifier input stage is based on two parallel-connected complementary differential pairs. Variations in the small-signal response are kept to a minimum by realizing an adequate shaping of the CM response of the input stage, while still reducing deviations in the total limiting current of the two input pairs with respect to traditional solutions. This is achieved independently of the gm-ID characteristic of the amplifier input devices and of any strict matching condition between the complementary input pairs. Experimental results from a 3-V 0.8-μm CMOS test-chip are given.

Transconductance enhancement in bulk-driven input stages

This paper presents two different circuit techniques which rely on partial positive feedback to enhance the effective transconductance of a CMOS bulk-driven input stage, thus leading to improved values for the DC gain and the gain-bandwidth product. The first approach modifies the conductance of the active load, while the second solution acts directly on the input differential pair of the transconductor. The suitability of the presented techniques is demonstrated by the design of operational transconductance amplifiers operating at two different supply voltages, i.e., 2.4 V and 1.0 V, in standard 0.35-mum CMOS technology. Simulated results are given.

1-V Quasi constant-g m input/output rail-to-rail CMOS op-amp

A CMOS operational amplifier with input/output rail-to-rail range is presented. The circuit operates with a 1-V single supply. It uses dynamically biased input level shifters, controlled by a novel tuning scheme, to extend the input common-mode voltage range from one rail to the other. The tuning circuit takes the total biasing current of the amplifier input stage and, hence, its small-signal response into account. As a result, the total input transconductance variations are lower than ±10% over the entire voltage range. Experimental results on a test-chip fabricated in standard 0.8-µm CMOS technology are given.