IN TODAY’S HIGH-SPEED communications systems, wideband amplifiers are created through a variety of techniques. Recently, engineers from Austria’s Technical University Vienna began working with designers from Infineon Technologies (Munich, Germany) on design considerations for inductorless broadband amplifiers based on the properties of the transistor in common collector configuration. This theory was then validated by a four-stage amplifier, which shows a measured 3-dB bandwidth of 84 GHz and a differential gain of 20 dB. The work was conducted by Saverio Trotta, Herbert Knapp, Klaus Aufinger, Thomas F. Meister, Josef Bock, Bernhard Dehlink, Werner Simbürger, and Arpad L. Scholtz.
This fully differential amplifier is based on a cascode topology. The cascode multiple-device configuration shows little high-frequency feedback through the base-collector capacitance with a very large 3-dB bandwidth. To broaden the cascode’s bandwidth, engineers can drive this stage with cascaded emitter-followers (EFs). Yet this approach leads to potential instability- especially when more than one EF pair is cascaded.
To avoid this instability, the proposed design methodology combines a cascode with a driving stage. In doing so, the researchers developed a broadband amplifier with very flat frequency response. The cascode stage’s bandwidth is further broadened by resistive emitter degeneration and a dynamic bias for the common base stage. The resulting silicon-germanium (SiGe) bipolar amplifier delivers a gain bandwidth product of more than 840 GHz. It consumes 990 mW of power at a supply of -5.5 V. See "An 84 GHz Bandwidth and 20 dB Gain Broadband Amplifier in SiGe Bipolar Technology," IEEE Journal of Solid-State Circuits, Oct. 2007, p. 2099.