Design of a Wideband dB-Linear Variable Gain Amplifier with Continuous Gain Adjusting in 90-nm CMOS Technology

This paper presents a wideband dB-linear variable-gain amplifier (VGA) with a DC-offset cancellation network in a 90-nm CMOS technology for high-speed applications. The VGA consists of a four-stage Cherry-Hooper amplifier, which is based on a feedback resistor and negative Miller capacitance to obtain wide bandwidth without inductors in the main circuit. A unique method for this structure to generate pseudo-exponential function is analyzed in detail. Therefore, an accurate dB-linear characteristic is realized in this VGA without any exponential generator circuits. The linear control voltage is applied directly to the gate of the transistor and this simple control method exhibits good robustness to process and temperature variation. Measurement results show that the voltage gain varies from -28 dB to 27 dB continuously with more than 4.3 GHz bandwidth and a dB-linear range of 32 dB with less than ±0.6 dB gain error. Without the output buffer, it consumes 8.5 mW from a 1.2V supply and occupies a small die area of only 0.04 mm2.