I. Introduction
The frequency generation is an important part of millimeter wave (mm-wave) and microwave wireless communication systems [1]. VCOs (voltage-controlled oscillators) generate high frequency signals directly by employing an oscillator tuned at the fundamental frequency. However, VCOs need high quality varactors for their performance, moreover, they are governed by constraints including low output power, phase noise, jitter, stability, and high cost. To abate the issues of VCOs, frequency multipliers are implemented along with VCOs for signal generation. The generation of signals at higher frequencies can be realized by frequency multipliers in conjunction with broadband amplifiers. Frequency multipliers employ a high-quality low frequency signal (< 20 GHz) for upscaling [2]. Active frequency multipliers are particularly used in mm-wave circuits, broadband wireless systems, and transceiver system applications, as they provide higher conversion gain, higher isolation, low phase noise, and multiplication without losing signal integrity. The advancement in high speed and high frequency operating transistors has allowed to exploit the frequencies above 30 GHz for modern wireless applications, phased array applications, imaging, spectroscopy, and automotive radar systems. Increasing speed and cutoff frequencies of the silicon based active transistors of the SiGe: BiCMOS technology offers lower cost, high output power, large scale integration and fmax values higher than 700 GHz [3]. Frequency multipliers using SiGe: BiCMOS HBTs have been reported in [4] [5] [6]. The single-ended frequency doublers are discouraged as they require filters or open-circuited stubs to suppress the fundamental harmonic tone, consequently, increasing the chip area and limiting the bandwidth of the overall circuit [7]. The balanced/differential configuration allows suppression of fundamental and odd-order harmonics without the need for filters.