Design and Analysis of a Wideband Gilbert Cell VGA in 0.25-InP DHBT Technology with DC-40-GHz Frequency Response

A differential variable gain amplifier (VGA) for wideband baseband signals has been designed, analyzed, and implemented in a 0.25-μm InP double heterojunction bipolar transistor technology with fT / fmax of 370/650 GHz. The 3-dB frequency bandwidth is measured to be 40 GHz with a maximum gain of 31 dB, resulting in a gain bandwidth product (GBP) of 1.4 THz, four times higher than previously reported GBP from a Gilbert cell-based VGAs. Furthermore, it measures a gain control range of 44 dB, a noise figure of 6.2 dB, an output third-order intercept point of 17 dBm, and a total power consumption of 350 mW from a single -7-V supply. With pseudorandom binary sequence test pattern signals, a clear open eye at 44 Gb/s was observed. The complete circuit, including on-chip integrated bias network and pads, measures 0.77 mm2. We analyze the VGA for the 3-dB bandwidth and GBP by the use of zero-value time constants method to analytically identify the maximum GBP with respect to the design parameters and current bias.