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The automatic gain control (AGC) circuit is a critical building block in modern wireless transceivers. It is used in both the transmitter (TX) and receiver (RX). In the TX, the AGC is used to regulate the output power level. For example, a well-known issue in a CDMA system is the problem of simultaneously receiving two signals of different strength. If two users transmit to the base station (BS) at the same time, the received power from the close user will be much higher and will jam the signal from the far away user if the power level difference is large. It is desirable to use an AGC to control each TX’s output power so that the power received from all users is equal. In the RX, the received signal can vary drastically in a mobile environment. At the same time, the analog-to-digital converter (ADC) requires a fixed input level. An AGC circuitry can be used to stabilize the received power level at the ADC’s input.

Original equipment manufacturers (OEM) demand highly integrated transceiver integrated circuits (IC) from RFIC vendors. CMOS technology has been the technology of choice because of its low cost and higher integration features. This article focuses on the AGC implementations in CMOS technology. AGC circuitry has been well studied and researched. This article presents seven widely used topologies for the AGC, including variable transconductance, variable degenerated feedback, variable biasing at the gate, variable biasing at the source, variable supply, variable feedback and variable T network. Each topology is discussed in detail.