Improved common-mode feedback circuit suitable for operational transconductance amplifiers with tuning

A simple modification is proposed for a popular common-mode feedback circuit used in operational amplifiers, such that it can be used in an operational transconductance amplifier (OTA) without leading to significant DC offset caused by the unavoidable tuning of the OTA. Simulation results show that the improved circuit has nearly zero DC offset for the whole tuning range.<>     

Efficient switched-capacitor common-mode feedback circuit for high-speed low-power amplifiers

A new switched-capacitor (SC) common-mode feedback (CMFB) circuit for fully-differential operational amplifiers (op-amps) is presented. By reducing the amplifier capacitive load with respect to conventional SC-CMFB schemes, the proposed solution guarantees a significant improvement of the op-amp speed performance. A typical SC integrator employing the new CMFB has also been designed in 0.35 μm CMOS technology. Simulation results show that, for a given power consumption, the op-amp settling time can be about halved by using the proposed CMFB instead of the conventional one.     

The design of fully differential CMOS operational amplifiers without extra common-mode feedback circuits

A new fully differential CMOS operational amplifier (op amp) without extra common-mode feedback (CMFB) circuit is proposed and analyzed. In this op amp, simple inversely connected current-mirror pairs are used as active loads. From the theoretical analysis, it is shown that the common-mode signal can be efficiently suppressed by the reduced effective common-mode resistance of the active load. The proposed op amp with 2 pF capacitance loadings has an open-loop unity-gain bandwidth of 63 MHz, a phase margin 47°, and a dc gain of 67 dB in 3.5µm p-well CMOS technology. The common-mode gain at a single output node can be as low as —38 dB without extra CMFB circuit. Experimental results have successfully confirmed the capability of the efficient common-mode rejection.This work was supported by the United Microelectronics Corporation (UMC), Republic of China, under Grant C80054.     

Differential amplifiers that reject common-mode currents

The transistor differential pair is analyzed and it is shown that the common mode rejection ratio is limited both in magnitude and bandwidth by device parameters. The performance is not significantly improved by series common-mode feedback. It is shown that amplifiers with shunt common-mode feedback reject common-mode input currents and that their common-mode rejection ratio can be made independent of device parameters. Applications of such amplifiers include differential operational amplifiers, rejection of large common-mode voltages, and transducer amplifiers. Basic circuitry is described and the performance of a practical circuit is given.     

Achieving wide-band common-mode rejection in differential amplifiers

A design method is presented for achieving wide-band common-mode rejection (CMR) in discrete or integrated differential amplifiers. The method, which is supported by experiment, shows that there is an optimum (noninfinite) value for the tail impedance, and that infinite rejection can be achieved at any frequency even in the asymmetrical case.     

Continuous-time common-mode feedback for high-speed switched-capacitor networks

This paper deals with the design of a continuous-time common-mode feedback (CMFB) for switched-capacitor networks. Its reduced input capacitance decreases the capacitive load at the output of the fully differential amplifier, improving its achievable gain-bandwidth (GBW) product and slew rate. This topology is more suitable for high-speed switched-capacitor applications when compared to a conventional switched-capacitor CMFB, enabling operation at higher clock frequencies. Additionally, it provides a superior rejection to the negative power supply noise (PSRR/sup -/). The performance of the CMFB is demonstrated in the implementation of a second-order 10.7-MHz bandpass switched-capacitor filter and compared with that of an identical filter using the conventional switched-capacitor CMFB (SC-CMFB). The filter using the continuous-time CMFB reduces the error due to finite GBW and slew rate to less than 1% for clock frequencies up to 72 MHz while providing a dynamic range of 59 dB and a PSRR/sup -/>22 dB. Both circuits were fabricated in 0.35-/spl mu/m CMOS technology.     

Finite common-mode rejection in fully differential operational amplifiers

A nonlinear mechanism which affects common-mode rejection (CMR) of fully differential integrated operational amplifiers (opamps) is highlighted and analysed by computer simulations. In particular, it is shown how the finite CMR of fully differential opamp circuits under practical operating conditions is mainly related to such a mechanism rather than to transistor mismatch.     

An improved common-mode feedback loop for the differential-difference amplifier

Unconditional stability of the high-gain amplifiers is a mandatory requirement for a reliable steady-state condition of time-discrete systems, especially for all blocks designed to sample-and-hold (S/H) circuits. Compared to differential path, the common-mode feedback loop is often affected by poles and zeros shifting that degrades the large signal response of the amplifiers. This drawback is made worse in some well-known topologies as the difference-differential amplifier (DDA) that shows non-constant transconductance and poor linearity. This work proposes a body-driven positive-feedback frequency compensation technique (BD-PFFC) to improve the linearity for precision DDA-based S/H applications. Theoretical calculations and circuit simulations carried out in a 0.13 μm process are also given to demonstrate its validity.     

Push-pull current circuit for biasing CMOS amplifiers with rail-to-rail input common-mode range

A push-pull current circuit for biasing CMOS amplifiers with rail-to-rail input common-mode range is presented. By means of a feedback action, the circuit avoids the large magnitude deviations inherent to this type of amplifier and thus facilitates their optimisation and compensation. Simulated and experimental results obtained from a fabricated 2 mu m CMOS test chip are reported.<>     

A fully balanced pseudo-differential OTA with common-mode feedforward and inherent common-mode feedback detector

A pseudo-differential fully balanced fully symmetric CMOS operational transconductance amplifier (OTA) architecture with inherent common-mode detection is proposed. Through judicious arrangement, the common-mode feedback circuit can be economically implemented. The OTA achieves a third harmonic distortion of -43 dB for 900 mV/sub pp/ at 30 MHz. The OTA, fabricated in 0.5-/spl mu/m CMOS process, is used to design a 100-MHz fourth-order linear phase filter. The measured filter's group delay ripple is 3% for frequencies up to 100 MHz, and the measured dynamic range is 45 dB for a total harmonic distortion of -46 dB. The filter consumes 42.9 mW per complex pole pair while operating from a /spl plusmn/1.65-V power supply.     

Capacitive feedback technique for wide-band amplifiers

A bandwidth-enhancing technique is presented for wideband amplifiers. In this technique a capacitive feedback scheme is used in analogy to resistive feedback amplifiers. As capacitive feedback does not lower gain, the technique does not trade off gain for bandwidth. Computer simulations and practical circuits show a considerable improvement over the conventional widebanding techniques     

Distortion in high-frequency FET amplifiers

A high-frequency nonlinear model of FET's is developed in which all sources of nonlinearities, including the output conductance, are accounted for. The distortion resulting from this model is represented using the Volterra series approach, and a high degree of accuracy between the theory and the measured results is obtained. Although the theory holds only for circuits that exhibit a slight deviation from linearity, it is found that fairly good agreement exists at larger signal levels. Harmonic, intermodulation, and cross-modulation distortion (CMD) that occur in a typical high-frequency single-stage FET amplifier are all analyzed and experimentally verified. It is observed that the CMD that occurs in the FET under consideration is of the AM type.     

Differential log-domain filters with high-gain common-mode feedback

A new class of differential log-domain filters based on common-mode feedback (CMFB) is demonstrated. Conditions for adding CMFB to differential log-domain integrators while preserving external linearity are derived. Because signals within differential log-domain circuits are nonlinear, the definition of "common-mode" in these circuits is not unique. Three possible definitions are explored, and each is illustrated with a simple but practical circuit. The circuits provide large CMFB loop gain, which forces common-mode signals to follow a desired nonlinear function. This allows closed-form expressions for the nonlinear internal signals to be derived, which is highly advantageous for design. Second-order filters using CMFB were compared to filters without CMFB through simulation and through measurements of fabricated filters. Adding CMFB is shown to allow greatly increased undistorted signal swings.     

Multiple-loop feedback filters using current feedback amplifiers

Novel high-order filter topologies realised by employing current feedback operational amplifiers as active elements are introduced in this article. These are constructed from multiple-loop feedback paths and could be utilised for the realisation of filters of arbitrary order and type. An offered benefit is the requirement for employing only grounded capacitors and resistors. Two design examples are provided and the correct operation of the corresponding topologies has been evaluated through experimental results.     

Referred impedance noise analysis for feedback amplifiers

A simple approach for calculating the equivalent input noise spectral density of feedback amplifiers is proposed. The method is particularly relevant to the design of amplifiers for use in optical-communication receivers.     

Current gain high-frequency CMOS operational amplifiers

The performance of high unity gain-bandwidth current gain-based CMOS operational amplifiers fabricated in a 1.5-/spl mu/m CMOS digital process is discussed. High unity-gain bandwidth was achieved by using short-channel MOS transistors operating in the current gain mode. Stacked current mirrors have been utilized as current gain stages to minimize the effects of the channel-length modulation in short-channel MOS transistors. Open-circuit gain of 60 to 70 dB, a unity-gain bandwidth of 70 to 100 MHz, and slew-rate of 200 V//spl mu/s were demonstrated at a DC power dissipation of 1-2 mW.     

A low-powerlow-voltage slew-rate enhancement circuit for two-stage operational amplifiers

正A novel circuit is presented in order to enhance the slew rate of two-stage operational amplifiers.The enhancer utilizes the class-AB input stage to improve current efficiency,while it works on an open loop with regard to the enhanced amplifier so that it has no effect on the stability of the amplifier.During the slewing period,the enhancer detects input differential voltage of the amplifier,and produces external enhancement currents for the amplifier,driving load capacitors to charge/discharge faster.Simulation results show that,fora large input step,the enhancerreduces settling time by nearly 50%.When the circuit is employed in a sample-and-hold circuit,it greatly improves the spur-free dynamic range by 44.6 dB and the total harmonic distortion by 43.9 dB.The proposed circuit is very suitable to operate under a low voltage(1.2 V or below) with a standby current of 200μA.     

Dynamic gain equalization in two-stage fiber amplifiers

Relative gain differences among channels in WDM-amplified lightwave systems can be corrected in two-stage fiber amplifiers having complementary gain spectra in each stage. With two channels spaced by 2.5 nm, relative gain corrections of 1 dB were demonstrated. Simulations show that this method can be used to dynamically equalize the channel gain in long-distance transmission systems