Design Of High-performance Switched-capacitor Variable Gain Amplifier

With the development of electronic technology, the application of variable gain amplifier(VGA) in wireless communication system and image processing system is becoming more and more widely. In order to receive and process the input signal better, A variable gain amplifier is usually needed to adjust the amplitude of the signal to meet the requirements of the subsequent-stage signal processing circuit. At present the research of the variable gain amplifier is towards the direction of high accuracy, high linearity, wide gain dynamic range and low offset.In this paper, a variable gain amplifier, which is applied to the analog front end of the image sensor readout circuit, is designed to achieve the amplification function and act as the S/H circuit of the subsequent-stage ADC. In order to meet the requirements of high accuracy, high linearity and large output range of the image sensor circuit, the VGA circuit uses the closed loop structure. A switched capacitor structure is presented as the feedback network, because it consumes small static current, moreover, the capacitor has better matching characteristics comparing with resistance in the industrial manufacturing. The whole circuit is mainly composed of a high performance operational amplifier, a capacitor feedback network and a clock generation circuit. An offset correction DAC is added to compensate the dark current generated by the image sensor. The gain boosting technique is introduced to the two-stage operational amplifier to maximize the open loop gain of the whole opamp, therefore, the VGA circuit can obtain very high accuracy. The split capacitor structure is used in the offset DAC to save chip area.In the background of the application of image sensor, a variable gain amplifier which has 14 bit sampling accuracy at 40MS/s sampling rate is studied and designed. The circuit is designed with 0.18μm CMOS(Complementary Metal Oxide Semiconductor) technology, at a 3.3V power supply, it consumes 65 mW of power, the area of the layout is 1850x950μm2. The common mode voltage of the circuit is 1.5V, and the input signal voltage range is from 1V to 2V. The experimental result shows that the VGA can achieve a dynamic range of-3dB to 19 dB with 256 gain steps. The offset correction DAC has ±8 bit resolutions and the output referred correction ranges and steps increase with the VGA gain, it can achieve a correction range of-507 mV to 507 mV with VGA gain 8x. Under the 17.06 MHz input signal, this VGA achieves an ENOB of 14.9 bit, an SNR of 91.4dB, an SFDR of 97.9dB. The results show that the circuit has the advantages of high degree of accuracy, high linearity, can meet the design requirements.