A Multi-Stage RC Compensation Technique for Decoupling the Transimpedance and BW: Creating High Speed and Low Noise TIA Designs

The gain and bandwidth of a shunt-feedback Transimpedance Amplifier (TIA) is limited by a so called transimpedance (TI) limit. This limit dictates the maximum possible value of the feedback resistance ( $R_{F}$ ) for a targeted bandwidth. Additionally, the input referred noise of such TIAs is inversely proportional to the $R_{F}$ , which presents a challenge in simultaneous optimization of bandwidth, noise and transimpedance gain. In this paper, the TI limit is revisited, and a multi-stage RC compensation technique is presented for the design of the open-loop amplifier for a closed-loop shunt-feedback-based TI stage. This paper shows that with the appropriate pole-zero positioning, the DC transimpedance gain can be decoupled from the closed-loop TI bandwidth. This is achieved by placing a zero in the open loop transfer function to reduce the impact of the closed loop dominant pole created by the input capacitance and the RF. As a result, without the need for area consuming inductors, a TI stage is realized which has a transimpedance limit that is larger than the conventionally assumed limit. Additionally, the proposed RC compensation network provides more control over the pole-zero positioning which results in smooth overall frequency response after equalization. This is verified by experimental results which show that the proposed technique achieves a much greater transimpedance gain as compared to that of the conventional limit while reducing the noise and without any significant deterioration of bandwidth. The design has been implemented in a 90 nm BiCMOS process from Global Foundries (GF-9HP). A detailed comparison of the proposed approach is presented with other TIA designs. As per the author’s best knowledge, the proposed design outperforms the state-of-the-art TIA designs in terms of the noise-transimpedance-bandwidth trade-off.