A Varactor-Based Balanced Biphase Variable Attenuator With a Novel Single Control Voltage Biasing Technique

This article presents a balanced biphase variable attenuator with a novel biasing technique that greatly simplifies its control. The variable attenuator consists of two identical reflection-type phase shifters operating symmetrically in a balanced configuration. The proposed technique combines a simple biasing circuit with a method to calculate optimal voltages for the varactor diodes, allowing control with a single voltage. This study demonstrates that despite the nonlinear phase response of the reflection-type phase shifters with respect to bias voltage, the proposed technique achieves perfect control of the variable attenuator with zero insertion phase variation when varactor diodes with a $\gamma $ value of 1 are used, where $\gamma $ is the slope of the log( $C_{v}$ ) curve. Furthermore, it shows that for $\gamma $ values close to or greater than 1, a nonzero phase error is observed, although it remains negligible. A sensitivity analysis of the balanced structure reveals that amplitude imbalance between the two branches affects the attenuation range and introduces phase variation, while phase imbalance only impacts the minimum attenuation. Experimental results show an attenuation range of 27.3 dB, a return loss of 25.9 dB, a phase variation of 24.4°, and a root-mean-square (rms) phase error of 4.7° at 2.45 GHz. From 2.33 to 2.57 GHz, the attenuation range is over 25 dB, the return loss is better than 21 dB, and the phase variation and rms phase error remain below 72.1° and 12°, respectively. With simplified control, zero power consumption, and possibility of energy reuse, this attenuator is well suited for energy-efficient systems.