Electronics designed to endure the cold of space

PORTLAND, Ore. — Spacecraft require heaters to protect their electronics from the cold, which for Moon missions can extend down to cryogenic temperatures below minus 150 degrees C (-238 F).

Eliminating the extra expense, weight and power consumption of "warm boxes" used to house electronics was the goal of University of Arkansas electrical engineers who presented their design for electronic building blocks that can function down to minus 180 degrees C at this week's IEEE Aerospace Conference (March 7-14, Big Sky, Mont.).

"Our device is designed specifically for extreme temperatures, including temperatures in the cryogenic region," said University of Arkansas (Fayetteville) EE professor Alan Mantooth.

The layout of the differential amplifier has three main segments: input and output stages, and their respective common-mode feedback circuits, all surrounded by the capacitors used for compensation.

The world's differential amplifier circuit designed specifically for temperatures extending down into the cryogenic region was fabricated using IBM's silicon-germanium BiCMOS process. Some of the designs have been tested as fully operational to below the temperature of empty space--2 degrees Kelvin (-271 degrees C or -456 F).

The differential amplifier circuit multiplies differences in voltages across its two inputs, rejecting any common mode signals like noise that were present on both inputs. To achieve low-temperature operation with a large differential gain over a wide frequency range the researchers used two common-mode feedback circuits to control the voltage of both the input and output stages independently.

Instead of NMOS transistors, which can trap a charge and exhibit hot carrier effects at cryogenic temperatures, only heterojunction bipolar npn and PMOS transistors were used in the design, since they exhibit the best temperature stability across the widest range.

The differential amplifier was divided into three main segments: input and output stages, and their respective common-mode feedback circuits--each surrounded by guard rings to provide isolation between stages for better immunity to noise, latch-up and radiation effects.

The current mirror and differential-pair transistors were carefully matched. Dummy transistors surround the cross-coupled polygate MOS transistors to combat irregular edges and insure the gate lengths of the PMOS devices are identical.

Using a 3.3-volt power supply with 100-microamps bias current, the BiCMOS differential amplifier achieved an open loop gain of 72 dB and a unity gain frequency of 130 MHz for an input common-mode range from 1 to 2.3 volts. Full functionality was tested and verified over the entire extended temperature range of 125 degrees C. down to minus 140 degrees C, with reduced performance that was nevertheless usable all the way down to minus 180 degrees C.