Abstract
SiGe has rapidly grown to a mature process technology addressing a broad array of applications. Most of the literature discussing these applications focuses on RF circuits and very high-speed data links; both take advantage of the high switching speed, low noise, and current handling capabilities afforded by SiGe transistors. The addition of germanium into a silicon bipolar junction transistor (BJT) technology allows the component engineer to optimize different parameters. These applications usually force the focus on Ft or Fmax. While these are also important in the design of precision circuits and or high linearity amplifiers, current gain (Hfe) and Early voltage (Va) are often design limitations that force use of more complex circuits. Additionally, usually only NPN transistors are available which can significantly complicate the design of critical analog building blocks. SiGe in fact provides significant performance boost to PNP transistors and in the last couple of decades complementary SiGe BJT processes are offered by several major semiconductor manufacturers. Although generally the focus has been on relatively low voltage devices with Bvceos of significantly less than 5V, SiGe can also be applied to the design of much higher voltage transistors with a significant boost in performance.
Many integrated circuits are built in such complementary SiGe Bipolar technologies and offer significant advantages over equivalent devices built in more conventional processes. To address low voltage applications, we will discuss how complementary SiGe can be applied to the design of a high-speed 16bit analog to digital convertor (ADC) and the design of a special buffer amplifier developed to condition the signal fed to such ADCs. Higher voltage (+/-15V supplies) precision amplifiers are still a significant market and have warranted the development of specialized complementary SiGe technologies. We will work through the design of a typical amplifier and show how the improved transistors lead to better end-product performance.
Feedback amplifiers are generally easier to stabilize if there are fewer gain stages. Compensation techniques are available to stabilize complex amplifier designs which generally sacrifice performance. The increased gain offered by SiGe BJTs compared to alternatives can reduce the number of gain stages needed to meet a specific specification. The availability of a complementary PNP transistor can also significantly simplify circuit design. The combination of the gain of the SiGe BJT (due to high Va and Hfe) together with the simplicity and symmetry offered by complementary bipolar circuit design enables the design of very high performance operational amplifiers.
We will detail an ampliier designed for very high performance portable audio applications. The combination of high Early Voltage and high Beta allows for a simple, single gain stage implementation, which results in low quiescent power, high capacitive load drive and high PSRR. The high beta and Ft of the SiGe transistors make this performance possible at low quiescent power.