Design of Digitally Controlled Variable Gain Amplifier in 130nm Process

Operational amplifier (opamp) are primary building blocks in analog and mixed-signal VLSI design. This project deals with the design of a two-stage opamp with a digitally controlled variable gain implemented through resistance adjustment in the feedback loop. Precise gain control through a digital interface is achieved while ensuring that the signal integrity over a wide frequency range is intact. Two-stage opamp are inherently unsta-ble as it contains two poles. Therefore, pole-splitting techniques are necessary to ensure stability and reduce ringing during step inputs. Miller compensation is used for extending the gain-bandwidth and for pushing the secondary pole further away from the dominant pole, thus increasing its phase margin and, as a result, bandwidth. A trim circuit inside the feedback loop allows fine adjustment of the gain without influencing the bandwidth. The circuit here demonstrates a very versatile variable gain amplifier for communication systems where often dynamic gain adjustment along with stable performance is necessary.