Julien Happich
EE Times Europe
Raytheon UK and Newcastle University have co-developed a silicon carbide based amplifier circuitry with operational amplifier like characteristics, exhibiting a high temperature gain of 200 at 400 °C.
The amplifier, once integrated and packaged into a single device, has the potential for use in monitoring and closed-loop control circuitry applications within a variety of harsh environment industries; such as aerospace, oil and gas, geothermal energy and nuclear.
“To date, the focus on Silicon Carbide semiconductors has been power electronics and exploiting the material’s ability to dissipate internally-generated heat,” says Dr Alton Horsfall, the Reader in Semiconductor Technology at Newcastle University. “For this project though we’ve focussed on creating circuitry that can operate in high temperature and other harsh environments. This could therefore lead to condition monitoring circuitry mounted on gas turbines or within the primary coolant loop of a nuclear reactor, which runs at about 350 °C.”
At the heart of the amplifier circuit is a lateral small-signal Junction Field Effect Transistor (JFET). This offers a significant improvement in reliability in hostile environments, because of the lack of a gate oxide layer. This results in a greater stability in the threshold voltage and a reduction in the intrinsic noise, making these structures well suited for the realisation of high temperature, low noise amplifier circuits. The current circuit is a fully differential, three stage amplifier, with a source follower final stage, optimised to operate on a ±15 V supply. Modifications enable voltage supplies of ±45 V to be utilised to increase the voltage headroom of the circuit.
Laboratory tests have shown the amplifier circuit has an open circuit gain in excess of 1500 at room temperature. The gain recording at 400 °C were limited by the passive components used in the circuit, according to Horsfall. This SiC monolithic integration of an Op Amp may lead to product commercialization for high temperature circuit designs.
