Contents I. Introduction
In radio frequency integrate circuits, the signals which received by antenna will varies greatly because of much outer factors influence. Therefore, variable gain amplifier(VGA) is used in the front of radio frequency circuits or intermediate frequency circuits. The VGA we designed in this paper mostly used in the tuner of digital TV Set- Top-Box. In order to achieve the control of gain change, we can utilize the MOS differential pair as the variable resistor for analog VGA because of lower-noise and lower-distortion [1]. However, when applied for low power situation, the resistances and parasitic capacitances would reduce the gain control range. The principle of three-order variable transconductance VGA is through changing the MOS bias current to obtain gain variation[2], which can realize the good dB-linear gain control. But the frequency response is bad at the low gain. To achieve low-noise, the multiplication factor N of MOS must be large enough, so the structure is not suitable for high-frequency operation. The VGA with Gilbert cell is widely used in RF circuits[3]. The input voltage will induce switching between the differential loads to control gain variation. Otherwise, the transconductance circuit converts the voltage signal into current signal. The advantages are low-noise low-distortion and high-frequency operation. The disadvantage is lacking a correlative circuit to carry out the linear-in-dB gain control, because of the mathematic relation between the current gain and control voltage. In this paper, we construct a simple circuit based on the Gilbert cell to realize the approximate exponential circuit, and the simulation results of VGA have better performance.
