Project: Active Filter

Transistor-level Design of a 1MHz Active Low-Pass Filter Circuit

Under Prof Anu Gupta | Course Project 2: Analog & Digital VLSI Design | with Hariharan Venkat & Anirudh Subramanian

For this project, we used the TSMC 180 nm node on LTSpice to implement a differential folded cascode operational amplifier, and then used it to implement a 1 MHz LPF circuit. The project report can be found here.

We started off by drawing the circuit topology provided, and deciding rough values for the bias voltages on paper. We then used SPICE to further fine-tune those values and achieve our gain requirement. We then went on to design the biasing circuitry as well.

However, the bandwidth was insufficient. We used series-shunt feedback and RC compensation to then trade gain for bandwidth. With our newly designed OpAmp, we proceeded to use it in feedback and implement the desired 1MHz active filter. We then examined parameters such as ICMR, PSRR and the like, and saw that our specifications had been met.

The project left us with a lot of learning outcomes. We learnt about the gm-Id method, switched capacitors, and about the challenges of analog layout design.