A Standard-Cell-Based CMFB for Fully Synthesizable OTAs

Abstract

In this paper, we propose a fully standard-cell-based common-mode feedback (CMFB) loop with an explicit voltage reference to improve the CMRR of pseudo-differential standard-cell-based amplifiers and to stabilize the dc output voltage. This latter feature allows robust biasing of operational transconductance amplifiers (OTAs) based on a cascade of such stages. A detailed analysis of the CMFB is reported to both provide insight into circuit behavior and to derive useful design guidelines. The proposed CMFB is then exploited to build a fully standard-cell OTA suitable for automatic place and route. Simulation results referring to the standard-cell library of a commercial 130 nm CMOS process illustrated a differential gain of 28.3 dB with a gain-bandwidth product of 15.4 MHz when driving a 1.5 pF load capacitance. The OTA exhibits good robustness under PVT and mismatch variations and achieves state-of-the-art FOMs also thanks to the limited area footprint.

A Novel Differential to Single-Ended Converter for Ultra-Low-Voltage Inverter-Based OTAs

“…7. As it can be observed, just [11] results in lower Area consumption and higher F OM S,A , F OM L,A . It has to be noted that also the DIGITAL OTA, presented in [2] results in low areaconsumption and good trade-off between performance and area consumption.…”

“…The same kind of differential to single-ended conversion is needed also in inverter-based or standard-cell-based OTAs exhibiting a differential input and a single-ended output. In recent research works dealing with inverter-based analog circuits such as [6], [7], [11], [29], [30], the differential to single-ended conversion is often performed exploiting the circuit topology depicted in Fig. 1.…”

“…Custom inverters allow degrees of freedom which are not guaranteed for inverter gates taken from standard-cell libraries. In fact, it is very difficult to control the bias point of standard-cell inverters and, as a result, the bandwidth, the power consumption and the phase margin of standardcell-based OTAs, compared with other not fully standardcell based architectures, exhibit a greater sensitivity to PVT variations [11]. In order to address these issues, several techniques to properly set node voltages have been proposed [7], and in some cases an improvement in terms of robustness with respect to PVT variations has been observed [11].…”

“…In fact, it is very difficult to control the bias point of standard-cell inverters and, as a result, the bandwidth, the power consumption and the phase margin of standardcell-based OTAs, compared with other not fully standardcell based architectures, exhibit a greater sensitivity to PVT variations [11]. In order to address these issues, several techniques to properly set node voltages have been proposed [7], and in some cases an improvement in terms of robustness with respect to PVT variations has been observed [11]. Another advantage of custom-defined inverters is that their body terminals are accessible and can be used to implement body-bias strategies which allow to enhance the robustness of such OTAs with respect to PVT variations.…”

“…In fact, the most common practice to design standard-cellbased analog functions is to exploit the inverter gate, taken from a standard-cell library, as a basic analog amplifier. Focusing on the inverter-based implementation of OTAs with differential input and single-ended output, the differential to single-ended (D2S) converter play a main role [3], [11], [29]; indeed its performance directly affects the overall commonmode rejection ratio (CMRR) and input common-mode range (ICMR) of the whole OTA. In recent literature [29], [30], inverter-based OTAs rely on a D2S topology based on an inverter driving another inverter with the input and output tight together which behaves as a "diode" connected device to implement a voltage gain approximately equal to −1.…”