As the final project for my Microelectronics and Devices class, I was tasked with building an Operational Transconductance Amplifier (OTA) using a 0.18 micrometer CMOS process that meets the specifications as shown below.
My topology for an OTA amplifier starts with an ideal current source at 10 microamps connected to a PMOS current mirror and a PMOS differential pair. This differential pair has a 500-millivolt AC component added to them with a 180-degree phase difference and connected to a common voltage that goes between 0 and 0.9 V. The different pair feeds into two biased NMOS transistors – the bias circuits being a diode transistor set up. The NMOS transistors have all the current from the first stage flowing into them. The second stage of my op-amp consists of the same topology as the tutorial. The body of all my PMOS transistors are connected to Vdd and the body of all my NMOS transistors are connected to ground excluding my PMOS differential pair whose body is connected to the source.
I unfortunately did not obtain a gain of 80 db for all common node voltages because the change in the common node voltage caused the node voltage of the differential pair to change. As that node voltage increased, the differential pair put more current into my most bottom NMOS pair. I attempted to fix the situation by decreasing the bias current into NMOS 5 and 2 but in the end, couldn’t regulate the current through that branch enough to keep a consistent gain of 80 db of more. I stilled retained a gain of 80 db for half of the common node voltage. I was able to achieve all other specifications for my OTA amplifier.
ES154: Microelectronics and Devices