Positive feedback technique and split-length transistors for DC-gain enhancement of two-stage op-amps

This study presents the design and simulation of a fully differential two-stage op-amp in a 0.18 μm complementary metal-oxide-semiconductor process with a 1.8 V supply voltage. In this op-amp, positive feedback technique and split-length transistors (SLTs) are employed to increase the DC-gain of the op-amp by about 22 dB without affecting the unity-gain bandwidth (UGBW), stability, power dissipation and output voltage swing of the conventional two-stage op-amp. A comprehensive analysis is provided for differential-mode gain, common-mode gain, power supply rejection ratio, input-referred noise, input offset, frequency response and the effect of using SLTs on DC-gain sensitivity. The proposed op-amp is utilised in a flip-around sample-and-hold amplifier (SHA). The output spectrum of the SHA shows the total harmonic distortion of 0.0023%. The post-layout and Monte Carlo simulation results show that the proposed op-amp has better performance than the state-of-the-art designs.

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