Chuck Bednar for redOrbit.com – @BednarChuck
As technology becomes more and more predominant in our lives, and the so-called “Internet of things” looks to connect everything from our homes and cars to appliances and medical devices, the challenge becomes creating more powerful transmitters to make this all possible.
For all of these smart devices and pieces of industrial equipment to exchange data, coordinate a variety of tasks and even help with their own maintenance, they must be equipped with chips that are powerful enough to broadcast to devices located several yards away, while still being energy-efficient enough to last for months or able to harvest power from renewable sources.
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“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,” Anantha Chandrakasan, an electrical engineering professor at MIT, said. He added that such devices need to be as energy-efficient as possible while not leaking power when not in use.
(Negative) charge the gate!
Chandrakasan and his colleagues are working on just such a transmitter – one which they claim can reduce off-state energy leakage 100-fold while still providing enough power to transmit data using a Bluetooth connection or even over longer-range 802.15.4 wireless-communication protocols. Their research will be presented this week at the Institute of Electrical and Electronics Engineers’ International Solid-State Circuits Conference (ISSCC 2015).
The professor explained that his team adapted similar techniques used to reduce power leakage in digital circuits for their new radio chip. Digital circuits feature transistors in which a pair of electrical leads are connected by silicon or another type of semiconducting material. While these materials are not great conductors in their native states, semiconductors used in transistors have a second wire placed on top of it which runs perpendicularly to the electrical leads.
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This wire is known as the gate, and sending a positive charge through it causes electrons to be drawn towards it, creating a bridge that allows currents to travel between the leads. However, the semiconductors are also not excellent insulators either, and even when no charge is applied to the gate, some current still leaks across the transistor. While this is a minute amount, over time it can significantly impact battery life in a device that is inactive most of the time.
The new radio chip developed by Chandrakasan and his colleagues reduces the amount of power leaked by applying a negative charge to the gate when the transmitter is idle. Doing so makes the semiconductor a better insulator by driving electrons away from the electrical leads, and tests of the chip found that the technique requires far less power than it saves by preventing leakage.
Pump, pump, the charge is pumpin’
The researchers explained that they use a circuit known as a charge pump in order to efficiently generate the negative charge. The charge pump is a small network of switches and charge-storing components known as capacitors, which collects the charge when the pump is exposed to the voltage which powers the chip.
Throwing a switch connects the positive end of the capacitor to the ground, causing a current to flow out the other end. The process is repeated frequently, and the only significant power drain associated with the process comes from throwing the switch, which occurs roughly 15 times per second.
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Chandrakasan and his colleagues also made the transistor more energy efficient by breaking down the generation of an electromagnetic signal into individual steps, only a few of which require higher voltages. For those parts of the process, their circuit uses capacitors and inductors to increase voltage locally, keeping the overall voltage of the circuit down while still enabling high-frequency transmissions.
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