Contents I. Introduction
In the Wideband Code-Division Multiple Access (WCDMA) wireless communication systems [1], the variable-gain amplifier (VGA) of mobile transmitters plays a fundamental role in optimizing system capacity. Since multiple users can share the same carrier frequency, the transmitter gain has to be regulated so that equal power is received at the base station from each user. In constant-envelope applications such as the Global System for Mobile Communications (GSM), output power control is usually implemented by adjusting the bias of the power amplifier (PA) [2]. However, this approach is not suitable for linear-modulation systems such as WCDMA because bias variations strongly affect the PA linearity. Therefore, an additional gain control block must be included in the system architecture. Moreover, the WCDMA standard requires a 74-dB output power control range, which is much higher than the 30-dB specification typical of the GSM system. Excellent temperature compensation is needed to accurately control the output power over such a wide dynamic range and a linear-in-dB characteristic [3] is preferred to minimize the resolution of the digital-to-analog converter driving the gain control terminal. Many successful implementations of variable-gain amplifiers have been reported over recent years, adopting either a discrete gain step approach (digital control signal) [4]–[7] or a continuously variable gain (analog control signal) [8]–[18]. Continuous-type VGAs are preferred in WCDMA systems to avoid signal phase discontinuities. Block diagram of the proposed VGA. Simplified schematic of the VGA core.
