Design of a dB-Linear Digitally-Controlled Variable Gain Amplifier

A decibel-linear digitally-controlled variable gain amplifier(VGA) was designed based on common-source amplifier with diode-connected load.The gain of the amplifier was controlled by simultaneously changing bias current ratio and transistor size ratio,which made current density of input/diode-connected transistors stay constant with gain variation and achieved significant improvement in linearity at low gains.The circuit was designed in NEC 0.35-μm CMOS process.Simulation results showed that the circuit had a dB-linear gain ranging from-11.85 dB to 11.64 dB with gain error less than 0.5 dB,a 1-dB compression point of 8.35 dBm for gain setting of-11.85 dB,and a-3 dB bandwidth above 62 MHz,which may vary between 62 MHz and 240 MHz depending on gain setting.