Design Challenges in HBT MMIC Amplifier Bias Circuits

During the development of a custom HBT MMIC, solutions will be evolved for the RF and DC circuits in parallel with the packaging and thermal design. The type of MMIC being developed will determine the relative difficulty these areas present. Following a brief introduction to HBT MMICs, this paper focuses on the issues associated with the design of the DC bias circuit, which need to be correctly addressed if the full performance capability of the process is to be unlocked. Merits and disadvantages of basic and advanced bias schemes are discussed for circuits targeting Class A through to Class AB operation. INTRODUCTION Hetero-junction Bipolar Transistor (HBT) Monolithic Microwave Integrated Circuit (MMIC) technology is firmly established and has been serving the RF & Microwave industry for many years. Simplistically, the technology can be considered as a means to integrate bipolar transistors, diodes, resistors, capacitors and inductors together on a single integrated circuit. A typical MMIC is shown in Figure 1 below. Figure 1: A plastic packaged MMIC (lid removed), with support components, on an FR4 PCB. The high useful operating frequency (GHz) and high operating current density (200μA/μm) features of the technology make it eminently suitable for realising a vast range of different products from low power multifunctional circuits through to power amplifiers. From a designer’s point of view, an important fact which distinguishes GaAs HBTs from traditional Silicon bipolar transistors, is that the ‘Vbe’ of these devices is somewhat higher at around 1.3V, and is subject to a negative temperature slope giving a variation of around ±0.1V over typical operating temperature ranges. Other than this, from an initial design perspective, HBTs can be thought of simply as very fast, high performance bipolar transistors. Finally, it is worth noting here, that standard processes only support NPN devices, which we will see is a major limitation in the scope of bias circuit design that is possible. APPLICATION CHALLENGES Any HBT MMIC development requires consideration of at least the following elements: