An active balun for high-CMRR IC design

Abstract

-A circuit topology that provides large bandwidth single-ended to differential conversion is presented. The proposed cell is based on a differential pair where feedback on common mode signal provides about 6 dB conversion gain increase, together with attenuation of common mode signal. Small-signal characterisation is presented, based on a block decomposition of the cell. Measurements on a SiGe test IC show more than 5 dB of gain improvement with respect to a simple differential pair.

A high CMRR BiCMOS active combiner balun

“…Phase and gain unbalance are less than 4° and 1.5 dB up to 5 GHz, respectively. These results are in the state of the art of active baluns [2–8].…”

“…Moreover, integrated passive baluns [1] showed very high losses and a narrow band behavior. To overcome these limitations, many active baluns were proposed in literature [2–8]. Baree and Robertson [2] proposed a distributed wideband balun.…”

“…However, the most popular structure is differential‐amplifier‐based active balun with high coupling impedance [5]. To enhance the amplifier performances, many techniques, as passive or active feedback [6, 7] have been proposed. Optimization of these topologies shows interesting performances but they operate only as splitter baluns and are not suitable for a combining function.…”

A wideband high-CMRR single-ended to differential converter

“…To validate the proposed topology and compare its performance with that of a differential pair driven single-ended, we have designed a test IC [15] using the STMicroelectronics BiCMOS7 technology [16], that features SiGe HBT's with a maximum f T in excess of 65 GHz. The IC contains three different test circuits, to allow de-embedding of measurements and comparison: two circuits are singleended to differential converters (S2D) with 50-X input and output buffers, as shown schematically in Fig.…”