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Analog/Mixed Signal

Twist On Negative Charge Enables IoT Transmitters

24 February 2015

Researchers are working on transmitters for Internet of Things machine-to-machine communications that are powerful enough to broadcast to devices dozens of yards away, but energy-efficient enough to last for months—or even to harvest energy from heat or mechanical vibrations.

"A key challenge is designing these circuits with extremely low standby power, because most of these devices are just sitting idling, waiting for some event to trigger a communication," said Anantha Chandrakasan, the Joseph F. and Nancy P. Keithley Professor in Electrical Engineering at MIT, in a statement. "When it's on, you want to be as efficient as possible, and when it's off, you want to really cut off the off-state power, the leakage power."

Chandrakasan's group recently presented a new transmitter design that reduces off-state leakage by 100-fold. At the same time, it provides adequate power for Bluetooth transmission, or for the even longer-range 802.15.4 wireless-communication protocol.

The researchers borrowed techniques used in digital circuits but with a twist.

Instead of using a positive charge, Chandrakasan’s group reduced the leakage by applying a negative charge to the gate when the transmitter is idle, driving electrons away from the electrical leads, making the semiconductor a much better insulator.

The MIT researchers used a charge pump circuit consisting of a small network of capacitors and switches. When the charge pump is exposed to the voltage that drives the chip, charge builds up in one of the capacitors. Throwing one of the switches connects the positive end of the capacitor to the ground, causing a current to flow out the other end. This process is repeated over and over. Throwing the switch, which happens about 15 times a second, is the only real power drain.

In tests conducted on a prototype chip fabricated through the Taiwan Semiconductor Manufacturing Company's research program, the MIT researchers found that their circuit spent only 20 picowatts of power to save 10,000 picowatts in leakage.

Ordinarily, the frequency at which a transmitter can broadcast is a function of its voltage. MIT researchers decomposed the electromagnetic signal into discrete steps, only some of which require higher voltages. For those steps, the circuit uses capacitors and inductors to increase voltage locally. The overall voltage of the circuit is kept low, while still enabling high-frequency transmissions.

If broadcasting the signal only every hour or so, the researchers' circuit can reduce power consumption 100-fold, providing an acceptable battery life.

This research was funded by Shell and Texas Instruments.

Questions or comments on this story? Contact nicolas.mokhoff@globalspec.com

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