Applications For A High Linearity Variable Gain Amplifier

Introduction

The high linearity VG025 variable gain amplifier (VGA) has recently been introduced by WJ Communications for use in transmitter and receiver automatic gain control (AGC) circuits, and as variable gain blocks following low noise amplifier stages in high dynamic range receiver front ends. The amplifier integrates a high performance amplifier and an analog attenuator within a single surface-mount package and is optimized for various current and next generation wireless technologies, such as GPRS, GSM, CDMA and W-CDMA, for which high dynamic range and small size are required.

The VG025 is a high dynamic range variable gain amplifier capable of achieving an analog attenuation range of up to 21 dB. Alternative solutions such as digital attenuators require much more complex driving circuitry and do not offer the precision that an analog attenuator has. Discrete configurations can also be undesirable because of the large number of components required and the large amount of space needed on a printed circuit board (PCB) assembly.

The amplifier has 1-dB compression point (P1dB) of 21 dBm and an output third order intercept point (OIP3) of 40 dBm. A key feature for the device is in its ability to maintain constant OIP3 and P1dB performance over the entire gain control range. This allows the VGA to be used in an RF chain without worries about system dynamic range degradation with varying attenuation levels. FET-based attenuation circuitry will rapidly degrade system linearity with increasing attenuation levels.

The amplifier can be adjusted to operate anywhere in the 50 to 2200 MHz frequency range using several off-chip components. This allows for flexibility for the amplifier to be used in both the RF and IF sections of a radio or transceiver. For frequencies of less than 1 GHz, the gain is approximately 15 dB with the gain control range being approximately 20 dB.

The VG025 is a multi-chip module utilizing a GaAs MESFET process for the amplifier and PIN diode technology for the attenuator. The high performance MESFET technology allows for minimal variation in gain over temperature. The gain variation range is controlled by a single 0 to 4.5V analog input requiring up to 28 mA for the full attenuation range. The VG025 operates from a 5V supply voltage with a typical bias current of 150 mA. Capable of operating over the -40 to 85 °C temperature range, the VGA achieves an MTTF greater than 1000 years at a mounting temperature of 85° C. The 16-pin 4x4 mm QFN package is lead-free/green/RoHS-compliant and is suitable for either standard tin-lead or higher temperature lead-free reflow profiles.

The internal configuration of the VGA allows various ways to drive the attenuator for different applications. The remainder of this paper will discuss the various types of applications where the high linearity VG025 can be used.