A low-power multi-gigabit CMOS/SIMOX LSI design using two power supply voltages

Ohtomo, Yusuke; Sawada, H.; Ohno, Toshikazu; Sakakibara, Yutaka; Sato, Yasuhiro; Ishihara, Tatsuya; Matsuoka, Satoshi; Shimaya, Masakazu

doi:10.1109/vlsic.1999.797223

Cited by 5 publications

(1 citation statement)

References 2 publications

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“…SOI devices have excellent RF performance even at voltages below 1V because of reduced capacitance in the drain region compared to bulk-CMOS technologies. Moreover, fully-depleted CMOS/SIMOX technology allows undoped MOSFETs with no process steps added to the normal digital CMOS/SIMOX LSI fabrication [1]. The undoped MOSFETs are normally-on transistors and can be made simply by masking in the channel-doping process.…”

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0.5-1 V 2 GHz RF front-end circuits in CMOS/SIMOX

Harada

Tsukahara²,

Yamada³

2000 IEEE International Solid-State Circuits Conference. Digest of Technical Papers (Cat. No.00CH37056)

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Minimizing supply voltage is one of the most effective ways to attain low-power RF circuits. These 2GHz receiver front-end blocks operate down to 0.5V. They are a low noise amplifier (LNA), a downconversion mixer, and a voltage-controlled oscillator (VCO) fabricated by 0.2µm fully-depleted CMOS/SIMOX technology, which is one of the thin-film Silicon-on-insulator (SOI) technologies. The 0.5V operation results from using two circuit techniques. One is an LC-tuned folded Gilbert cell, and the other is the use of undoped MOSFETs in the VCO core and buffers for the mixer and VCO. SOI devices have excellent RF performance even at voltages below 1V because of reduced capacitance in the drain region compared to bulk-CMOS technologies. Moreover, fully-depleted CMOS/SIMOX technology allows undoped MOSFETs with no process steps added to the normal digital CMOS/SIMOX LSI fabrication [1]. The undoped MOSFETs are normally-on transistors and can be made simply by masking in the channel-doping process. Thus, they have the same structure as normal (doped) ones except for the impurity concentration in their channel region. They do not show punch-through current even in short-channel devices. This is because the potential of their thin-film channel region is perfectly controlled by the gate. The undoped MOSFETs are effective in designing RF circuits that operate at 0.5V.To reduce supply voltage, transistor stacking must be avoided. A Gilbert cell, most commonly used for the mixer, normally has a stack of three transistors. The number of transistors in the stack is decreased by folding and then connecting the node at which the circuit is folded to a current source. Stacking of transistors in the folded circuit, however, cannot be avoided because the current source is usually made of a transistor. The dc drop in this transistor strongly reduces the operation range at supply voltages below 1V. Using a LC tank instead of the current source makes the dc drop zero. The LC tank acts as a current source at around its resonant frequency because the impedance is high. As a result, the stacking of MOSFETs is avoided. Figure 23.2.1a shows the schematic of the folded mixer. Using LC tanks having ~2MHz resonant frequency, the RF input pair (M2 and M3) is folded. If a Gilbert cell is simply folded with the dc current of each transistor conserved, the total current is doubled compared to that of the previous Gilbert cell, which results in the same power consumption. The total current, however, can be decreased by optimizing the size of the nFETs and pFETs because their dc operating points can be independently changed without affecting each other. Since the conversion gain is made in the RF input pair (M2 and M3) and the local oscillator (LO) input pairs (M4 -M7) only act as switches, current in the LO pairs can be saved. Total current of the folded mixer can be almost the same as that of a conventional Gilbert cell before it is folded.It is necessary to use a source follower as a buffer to make the output impedance of the mixer low enough. At extr...

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