Wideband high-isolation SPDT RF switch in 0.18-μm SiGe BiCMOS technology

A single pole double throw (SPDT) RF switch working in wide band is presented. The SPDT RF switch composed of many single pole single throw (SPST) switches changes the signal flow corresponding to control signals. It is very important to obtain low insertion loss and high power driving capability for dealing with the input signal of wide dynamic range. High isolation is also desirable for suppressing leakage of unwanted signal going through the off-state path. To achieve low insertion loss, the heterojunction bipolar transistor (HBT) along with MOSFET in SiGe BiCMOS process is employed. HBT shows inherently considerably high impedance between collector and emitter terminals when it turns off, so that RF signals leak less through the off-state path. The insertion loss of the proposed SPDT RF switch is maintained less than 2 dB over frequency range of interest. This RF switch cancels leakage signals coming out of the off-state paths, leading to isolation improvement. For leakage cancellation, transformer and common-base structure are employed. In addition, SPST switches are adopted in parallel with inductors for decreasing the phase deviation of the passing signal. Since the RF switch using MOSFET shows worse isolation as frequency increases, leakage signal cancellation is designed to work in higher frequency region for obtaining wideband operation. A high isolation is obtained over 23 dB in measurement. This RF switch operates from 1 to 8 GHz providing low insertion loss. In this design, 0.18-m SiGe BiCMOS process is used and the proposed RF switch occupies the area of 0.3 mm.