| Operational amplifier, a generic and important module in linear circuits, is widely used in majority analog and mixed-signal systems. operational amplifier are required to drive different capacitive loads in different applications, such as analog filters, high-resolution ADC(analog to digital converter,ADC), headphone drivers and LCD(Liquid Crystal Display, LCD) drivers. A single stage amplifier can such capacitive loads and is unconditionally stable, as channel length and supply voltage are continuously scaled down, the gain of the single stage amplifier is not sufficient for most of high resolution applications. Multistage amplifiers(two-stage or three-stage amplifier) have received much more attention due to the advantages of high gain and wide output swing. However, along with the introduction of the extra stages, the issue of frequency compensation arises since all multistage amplifiers suffer from close-loop stability problem due to the presence of multiple poles nature.The frequency compensation technique for two-stage and three-stage amplifier is researched in this thesis. Analyzing frequency compensation technique of state-of-the-art and their merits and faults, some frequency compensation strategies are proposed to improve the performance of the amplifiers. The research content of this dissertation is listed as below:(1)The frequency compensation technique for two-stage amplifier is analyzed and an embedded current amplifier compensation for two-stage amplifier is proposed. The current amplifier is embedded into the first stage. The non-dominant poles determined by the load capacitor are pushed to higher frequencies, which results in enhanced capacity of driving capacitive loads. Compared to the miller compensation with current amplifier, the GBW(gain-bandwidth product) achieved is increased. Furthermore, no extra bias circuit is required and the system offset is minimized due to the good symmetry of the first stage. To reduce the size of the compensation capacitor, a novel capacitor multiplier technique is proposed. Two voltage-controlled current sources are inserted in the proposed capacitor multiplier which is embedded into the first stage of the amplifier. The gain factor of the multiplier is enhanced. The novel capacitor multiplier technique is adopted in the two-stage amplifier, in which a small capacitor can be used to drive large capacitive loads.(2)On the basis of traditional two-stage amplifier, a split compensation for inverter-based two-stage amplifier is presented. The GBW of two-stage amplifier is limited by the transconductance of the input stage, both of the transconductances of the second stage are doubled due to the inverter-based stage structure and the gain and GBW are improved.(3) Frequency compensation large capacitive load low-power three-stage amplifier is presented in this paper. The capacity of driving capacitive load is limited due to the presence of the second miller capacitor, a SCCAC(single capacitor with current amplifier compensation) for large capacitive load three-stage is proposed, in which a current amplifier is inserted to separate the compensation capacitor from output of the first stage. The GBW of the internal feedback loop is increased to extend the ability of driving capacity. In addition, a Single capacitor with embedded current amplifier compensation(SCECA) for three-stage amplifiers with large capacitive loads is proposed, in which the current amplifier is embedded in the first stage, compared to the SCCAC amplifier, the gain of the amplifier is increased, which compensate the reduction of the dominate pole, the GBW is not affected as s result.(4)Compensation technique for low-power three-stage amplifier with wide range of capacitive loads. A number of frequency compensation strategies are proposed to extend the large loading capability, however they are exposed to instability when loaded by small capacitive loads. So a dual miller parallel compensation(DMPC) strategy is proposed, in which two compensation networks are added between the first and third stage. One compensation network is used to improve the large loading capability whereas the other one is used to guarantee the stability when loaded by small capacitive loads. So the DMPC amplifier can drive a wide range of capacitive loads. Furthermore, the concept of embedded current amplifier is employed to reduce the compensation capacitor required in dual compensation with embedded current amplifier(DCECA). The DCECA three-stage amplifier can improve the performance such as gain, GBW and slew rate.
|
PDF Full Text Request