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A CMOS single supply operational transconductance amplifier (OTA) is reported. It has high power supply rejection capabilities over the entire gain bandwidth. The OTA is fabricated using the AMI 0.5 μm CMOS process. Measurements show power supply rejection ratio (PSRR) of 120 dB to 10 kHz. At 10 MHz, PSRR is 40 dB. The high performance PSRR is achieved using a high impedance current source and two noise reduction techniques. The OTA offers very low current consumption of 25 μA from a 3.3 V supply. It is suitable for applications such as low drop out voltage regulators.
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Introduction: Recent trends of system-on-chip (SOC) design of modern integrated circuits have employed analogue building blocks to be placed on the same die together with a noisy digital, switched capacitor, and DC-DC converter circuitry, thus suffering from noise on the power supply lines. This noise can significantly decrease performance by reducing the dynamic range of the whole system, especially in high precision systems or if the circuits that are sensitive to supply noise are at the very beginning of the power supply/reference chain. Therefore, analogue circuits, such as operational amplifiers (OAs) that are used as voltage reference buffers or as on chip low dropout regulators, should have supply noise rejecting capabilities both at DC and at higher frequency.
A number of different techniques have been developed to overcome the problem of power supply rejection for OAs [1 - 3]. One technique is the insertion of a cascode transistor [1] between the compensation capacitor and the gate of the output transistor to prevent it from getting diode connected at high frequency. This technique offers a much improved high frequency power supply rejection ratio (PSRR) but complicates the compensation of the amplifier. Another method involves using a parallel path [2] from the supply lines to the output to cancel out the noise coming from the supply at the frequency...