Design goal

The OTA is an abbreviation for operational transconductance amplifier. Which is an operational amplifier with no buffer stage at the end because it drives a capacitive load.

The goal of this project is to design a fully differential folded cascode OTA with a common mode feedback network. this circuit can be used as a building block in larger systems such as active filters and data converters.

The fully differential folded cascode OTA is chosen for the following reasons:

• Larger output swing than telescopic cascode OTA.
• Flexible common-mode input range.
• The current consumption is nearly equal to the two-stage OTA but it has a better frequency response.

The Common-mode feedback network is used to stabilize the output common-mode voltage without affecting the differential mode operation. The CMFB loop senses the CM output level and compares it with the specified reference voltage value, then adjusts the bias voltage of the amplifier to correct the error and force the output voltage to the specified reference value.

Target specifications

Note: The specs can be changed to fit in any bigger system.

OTA schematic

Q1,2: (The input pair transistors) Operate as voltage to current converters.

Q3,4: Operate as current sources for the cascode branches.

Q5,6: Operate as a common gate stage to act as a current buffer.

Q7-10: Act as a high impedance load to get high voltage gain.

Q11:Acts as a current source and it's biased using the output of the common-mode feedback network. 

Common-mode feedback network schematic

Feedback loop

The circuit is placed in a capacitive feedback loop to obtain a gain of 2.

Team members

• Ahmed Abosheir
• Ali Waheed
• Laila Hany Fadl
• Mohamed ELsenary
• Mostafa Mohammad
• Omar Ali Mohammed Bakheet
• Youssef Mohammed

References 

[1] Dr. Hesham Omran's "Analog IC Design" course material.

[2] B. Razavi, “Design of Analog CMOS Integrated Circuits,” McGraw-Hill, 2nd ed,2017.