A low-distortion CMOS IF VGA with linear-in-dB control and temperature compensation

Abstract

A CMOS intermediate-frequency (IF) variablegain amplifier (VGA) is presented in this paper. A transconductance linearization scheme is proposed for the VGA core based on a signal-subtracting structure to achieve low distortion. Temperature-independent decibel-linear gain control characteristic is achieved by an exponential voltage generator based on transfer characteristics of differential pair. The whole VGA, including a highly-linear output stage, is fabricated in 0.25 µm CMOS technology. Measurements show that the VGA provides a total gain control range of 43 dB with less than 1.2 dB error over 0-80 • C, and a constant 3-dB bandwidth of 100 MHz. The third-order intermodulation (IM3) distortion at differential output of 2 V PP is better than − 55 dB. The VGA dissipates 22.6 mA averagely from 3.3 V supply, and occupies approximately 0.53 mm 2 .

1.5-V CMOS exponential current generator

“…Table 2 summarizes the main performance of the proposed exponential current generator and also presents a comparison between this work and other references. As it can be observed, other competitive alternatives [11,14] operate at higher supply voltages, but with lower threshold voltage values as it is depicted by the used technology processes. Only the circuit in [16] operates at 0.9 V with smaller linearity error, but it exhibits only the half dynamic control range and its functional behavior is based on pseudo-exponential approximation, compared with this work.…”

“…Unfortunately, the usage of the exponential input characteristics of bipolar or MOS in weak inversion has the disadvantage of the strong temperature dependency, which is mainly caused by the thermal voltage [4,7,[11][12][13][14]. Therefore, regardless to design method, an accurate and stable exponential generator imposes temperature compensation.…”