Robert Kolm

Efficient four-stage frequency compensation for low-voltage amplifiers

This paper proposes a simple and efficient frequency compensation scheme, targeted at low-voltage amplifiers. An amplifier is correspondingly designed in 0.12-mum standard digital CMOS technology. When the supply voltage is IV and the load is 500 pF, post-layout simulation results show that the amplifier attains a gain-bandwidth product of 40.2 MHz, an average slew-rate of 17.52 V/mus and a current consumption of 1.4 mA. The stability of the amplifier is confirmed by sufficient phase margin and fast settling behavior. The efficiency of the proposed compensation approach is also highlighted by a comparison of figures of merit (FOM) with recent designs. In addition, four new FOMs are developed to evaluate the compensation efficiency in a more accurate manner.

A low-voltage low-power fully differential rail-to-rail input/output opamp in 65-nm CMOS

A new fully differential rail-to-rail input/output opamp with constant small- and large-signal behavior fabricated in 65-nm digital CMOS technology is proposed. A novel current- mode common-mode feedback circuit is introduced to regulate the signal behavior. Measurements are done when the supply voltage is plusmn0.5 V and the load is 15 pF. The opamp attains a DC gain of 100 dB, a unity-gain frequency of 40 MHz, a phase margin of 64deg and a low power consumption of 720 muW. The small-signal behavior variation is only plusmn2.2%. The large-signal behavior is also verified to be constant.

A 3rd-Order 235MHz Low-Pass gmC-Filter in 120nm CMOS

A 3rd-order continuous-time filter with a cut-off frequency of 235MHz in 120nm CMOS is presented. A linearization technique of an operational transconductance amplifier (OTA) is proposed. The gain of this filter is digitally programmable between 0.5dB and -5dB. The third-harmonic distortion of this filter is -49dB in maximum gain for a 400mV peak-to-peak differential output signal. Two identical filters were implemented on one chip to measure the mismatch which is lower than 0.7 dB in amplitude and 7deg in phase

A linear transconductor and its application in an analog filter in 120nm CMOS

A gmC-filter in 120nm CMOS with a digital programmable gain and cut-off-frequency is presented. For this application a technique to enhance the linearity of a fully differential operational transconductance amplifier (OTA) is proposed. A filter of 3rd order with a cut-off-frequency between 235MHz and 285 MHz and a gain between 0dB and 5dB is implemented. The third harmonic distortion of this filter is -43dB for a 400mV peak-to-peak differential output signal for minimum gain and -48dB for maximum gain

Current-Mode Filters

Voltage-mode filters and especially their linearity and dynamic range suffer from the low supply voltages of deep-sub-micron and nanometer CMOS. This fact served as motivation to investigate current-mode filters with the hope for a better linearity and larger dynamic range. Current-mode filters also may allow saving of current-to-voltage and voltage-to-current converters compared to the case when voltage mode filters are used in combination with current-mode mixer and current-steering digital-analog converter for instance. A lower power consumption then should be obtainable. This chapter will give on overview on current-mode filters described in literature and will introduce new current-mode filters designed in 120 nm CMOS and 65 nm low-power CMOS. Their properties are described in detail.

Operational Transconductance Amplifiers (OTAs)

This chapter describes operational transconductance amplifiers (OTAs) and their linearization for use in g m -C filters. Super-source-follower and digitally programmable OTAs are detailed. The common-mode feedback used in these OTAs and a buffer amplifier are described. A OTA implemented in 120 nm CMOS is introduced inclusive measured results.

Operational Amplifier RC Low-Pass Filter

Several applications pose challenges for wireless receivers due to close blockers in the frequency spectrum. This requires amplifiers and filters with a high linearity. Due to the challenges of the nanometer hell of physics concerning design of operational amplifiers, new circuit architectures will be investigated. In fact, it will turn out that the low supply voltage of nanometer CMOS circuits is the most limiting factor to achieve a good linearity and a large dynamic range. A high-voltage operational amplifier finally will show the best performance in a filter-mixer combination. In addition, 1/f noise will turn out to reduce the choice of usable mixer topologies considerably.

G m -C Filters

Analog filters are required in many mixed-signal systems. They can be used for anti-aliasing purposes, before signals are sampled in an A/D converter or they can be used for reconstructing the signal after a D/A converter. Also at the output of a mixer a filter which suppresses out-of-band interferer signals is necessary. In general, when a filter is designed it is necessary to consider the whole system in which the filter is embedded. Important parameters for filters are in-band and also out-of-band distortions. A high in-band linear dynamic range avoids intermodulation between two in-band signals. Out-of-band distortions are also essential because they describe the immunity against some blockers or disturbances to other channels in a multi-band system. Other important parameters are the noise spectral density or the integrated noise. For some applications the out-of-band spectral noise density should be under a certain level, otherwise the noise would interfere to a neighboring channel. Sometimes, when the same filter is used in the I-path and in the Q-path of a receiver, the mismatch between these two filters should not be too high. G m -C filter topologies, a figure of merit, the state-of-art, the requirements for UWB and three implemented filters are described in this chapter.

Current-mode filter in 65nm CMOS for a software-radio application

For the transmit path of a software radio defined system a 3rd-order current-mode filter in 65 nm CMOS is presented. This filter consists of a 1st-order current-mode filter where a capacitance multiplication technique is applied. With this technique the needed capacitance is decreased by a factor of 2. The 2nd-order filter consists of an active gm-RC filter where the output voltage is also converted into a current. The filter has a switchable cut-off frequency of 1 MHz and 4 MHz, the current consumption is 5.26 mA at VDD=1.2 V and for an input current of 340 muA the THD is 1%.

Verzerrungsarmes gmC-Filter 3. Ordnung in 120 nm CMOS.

Um aktive analoge Filter im Frequenzbereich mehrerer hundert MHz realisieren zu können, bedient man sich der gmC-Topologie. Dabei werden Transkonduktanzverstärker (OTA) kapazitiv belastet, um einen exakt definierten Pol zu erhalten. Der Nachteil von gmC-Filtern ist insbesondere in der z. B. für Ultra-Wide-Band-Anwendungen erforderlichen Deep-Submicron-Technologie, dass die erreichbare Linearität gering ist. Zu diesem Zweck wird eine Linearisierung eines OTAs vorgestellt, die auch für große Eingangsdifferenzspannungen noch eine akzeptable Linearität erlaubt. Dieses Konzept wird dann für ein analoges Filter 3. Ordnung mit einer Grenzfrequenz von 235 MHz angewendet. Die Verstärkung dieses Filters ist in acht Schritten digital umschaltbar.For the realization of filters in the hundred-MHz frequency range the gmC-topology is a good candidate. To obtain an exactly defined pole the load of the operational transconductance amplifier (OTA) is a capacitor. The poor linearity is a disadvantage of a gmC-structure in ultra-wide-band applied deep-submicron technologies. This can be relieved by using a linearization technique which provides an acceptable linearity also for large input signals. This concept is applied for a 3rd-order filter with a cut-off frequency of 235 MHz where the gain is switchable in 8 different steps.