Bias sequencing is something that is very familiar to many RF/microwave engineers. Essentially, sequencing circuits are needed to ensure that bias voltages are applied in the correct order. However, a tech brief from Custom MMIC titled “Throw out complex bias sequencers along with the negative voltage supply” explains how sequencing circuits can for all intents and purposes be eliminated from the design process.
Amplifiers based on depletion-mode pseudomorphic-high-electron-mobility-transistor (D-pHEMT) technology have traditionally required bias sequencing circuits. As stated, sequencing circuits are utilized to properly bias transistors. Incorrect biasing often results in damage to a transistor. Furthermore, a depletion-mode device must also be powered down with the same sequence in reverse.
The situation is even more complicated for complex systems that contain multiple amplifiers based on depletion-mode technology. One specific example mentioned is a phased-array radar system in which sequencing must be applied to hundreds (or even thousands) of amplifiers. Any delays or offsets in the biasing scheme could have a major impact on the radar system’s overall sensitivity.
The tech brief explains that the solution to the problem lies in physics. Unlike D-pHEMT devices, enhancement-mode pseudomorphic-high-electron-mobility-transistor (E-pHEMT) devices are normally non-conductive and only sink current when both the drain and gate are biased—regardless of the sequence. Thus, sequencing circuitry is not needed for E-pHEMT devices. Furthermore, E-pHEMT transistors achieve performance levels that are comparable with (or even better than) D-pHEMT transistors, according to the tech brief. Specifically, in some instances, E-pHEMT devices can outperform D-pHEMT devices in terms of maximum gain, noise figure, and linearity.
Eliminating the need for bias sequencing can result in a number of benefits, such as a reduced bill of materials (BOM), more simplified circuitry, and a reduced number of extraneous noise sources. Designers can therefore focus on other important aspects of the system design. The tech brief concludes by stating that a number of manufacturers are utilizing E-pHEMT technology, with Custom MMIC currently offering a range of power amplifiers (PAs) and low-noise amplifiers (LNAs) based on E-pHEMT technology.
Custom MMIC, 300 Apollo Dr., Chelmsford, MA 01824; 978-467-4290
