Design and Analysis of a Telescopic OTA-Based Variable Gain Amplifier as a Adaptive Gain Controller

Author

Dungala Aditya, Chittipolu Vishnu Vardhan, Bhukya Eswar Nayak


Abstract

 This paper presents the design and simulation-based characterization of a variable gain amplifier (VGA) built around a fully differential telescopic operational transconductance amplifier (OTA) whose gain is set through the gate voltage of its tail transistor, together with an automatic gain control (AGC) loop formed by a MOSFET envelope detector and a differential error amplifier. The circuit is intended for the VGA-I and VGA-Q branches of a 28-GHz vector modulator. Gain-control relations are derived, and it is shown that the sign of the control characteristic must be evaluated at the carrier frequency rather than at DC for the loop to provide negative feedback. Cadence Virtuoso AC simulations of the OTA give a low-frequency differential gain of approximately 74 dB and a gain of 24.99 dB at 28 GHz. Process-corner sweeps (FF, FS, SS, SF) show a gain spread of 4.1 dB at low frequency that narrows to 1.6 dB at 28 GHz, whereas at the FF corner the gain depends strongly on temperature: the low-frequency gain is 2.0, 19.0 and 18.6 dB and the gain at 28 GHz is 1.0, 15.0 and 9.3 dB at −24, 27 and 55 °C. Transistor-width sweeps from 120 nm to 360 nm raise the gain monotonically. An open-loop gain-control sweep moves the 28-GHz gain between 20.1 dB and 31.9 dB over four control settings. The temperature sensitivity at the carrier motivates closed-loop regulation; closed-loop transient verification, the control-voltage-to-gain characteristic, power, noise and linearity are identified as remaining work.


Keywords

automatic gain control, envelope detector, operational transconductance amplifier, process corners, telescopic cascode, variable gain amplifier, 28 GHz.



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