A High-Linearity Constant-Bandwidth PVT-Tolerant 10 Gbps PAM-4 Inverter-Based Compact VGA

This work presents an inverter-based variable gain amplifier (VGA) with robust performance across process corners. The proposed VGA is designed for use in a PAM-4 optical receiver, where high linearity is critical. It employs an adjustable number of inverters to form a transconductance stage while the load is fixed to maintain a constant bandwidth. An indirect Miller-compensated low dropout regulator (LDO) regulates the operating point of the VGA, utilizing a replica inverter reference to ensure adaptable supply operation. This setup maintains consistent DC currents across process, voltage, and temperature (PVT) variations, thereby ensuring high linearity. The VGA achieves a bandwidth of 5.2 GHz across all gain settings, ranging from 0.5 dB to 8 dB in 0.5 dB increments and a gain error of less than ± 0.15 dB. For a 1 GHz sinusoidal input with an amplitude of 100 mV, the third-order harmonic distortion (HD3) is -55.63 dB at the minimum gain and -51.37 dB at the maximum gain, while the total harmonic distortion (THD) is 0.18% at the minimum gain and 0.27% at the maximum gain. The input-referred noise (IRN) at 1 GHz with the maximum gain is 1.2 5 nV/ SHz. The eye diagrams additionally demonstrate the linearity and noise performances. The VGA core consumes a maximum power of 5.28 mW from a 1.8 V supply and occupies a compact layout area of only 66 μ m 25 μ m. The circuit uses a commercial 65 nm CMOS process and operates with a 1.8 V supply.