| In semiconductor, the size for the gate channel length of MOSFET will be continued to scale down further in the near future for technological benefits. However, serious problems are posed for analog circuits. Scaling down devices reduces both output resistance and transconductance of the MOSFET. Consequently, it lowers intrinsic gain of MOS transistors and small-signal gain of diff-amp circuits. Besides that, scaling down will cause MOS devices becoming more sensitive to process-voltage-temperature (PVT) variations.;This dissertation presents a differential amplifier design with gain-enhancement using positive feedback that will address these two problems. The circuit is designed with short-channel MOSFETs, low power, and low voltage, resulting in small-signal voltage gain improvement over a conventional complementary-metal-oxide-semiconductor (CMOS) diff-amp and comparable to known published diff-amp circuits. Also, comparing with the standard CMOS differential amplifier and known published diff-amps, the proposed circuit has an improved output swing. In addition, the new circuit has a built-in tuning capability for adjustable gain or tuning out the process-voltage-temperature variations.;Although the proposed diff-amp circuit has advantages, it has some potential drawbacks such as positive feedback, lower unity-gain-frequency, and higher noise. Positive feedback is probably among the most concern for many because it is not widely used in circuit design. However, the new proposed circuit can be useful if the positive feedback is designed properly. Also, several positive feedback gain-enhancement circuits have been found in the published literature.;In addition, four two-stage fully differential op-amps are designed and compared in this dissertation. The first gain stages are implemented with a basic diff-amp, two known published diff-amps, and a new proposed diff-amp using positive feedback. The second stages are set up with a pair of common-source amplifier. The design op-amps are fully characterized with LT SPICE simulations. An op-amp design with a new proposed diff-amp shows good improvements on DC gain and positive power supply rejection ratio (PSRR+). However, some significant disadvantages, which are higher power consumption, higher effects of device mismatch, low common-mode rejection ratio (CMRR) and higher noise, are found on the op-amp designed with a newly proposed diff-amp. For open-loop gain, input common-mode range (ICMR), and output swing, all op-amps were comparable.;Lastly, the subject of frequency response is reviewed, and frequency compensation including pole splitting and zero cancelling techniques are implemented to keep the op-amp circuit stable and also have higher bandwidth.;Keywords: analog, gain-enhancement, differential amplifier, open-loop gain, cascode, positive feedback, PVT, tunable gain, voltage swing, fully differential op-amp, noise, harmonic distortion, unity-gain frequency, DC gain, power consumption, ICMR, output current, SR, offset voltage, output swing, PSRR, CMRR, CMFB. |
