FIG. 13 shows another embodiment of a phase shifting circuit. The input signal Vin is phase shifted via a control voltage, Vphase_mod. In this example, an enhancement mode FET, U2A, is included. An amplifier U1A provides coupling signal from the drain of the FET, U2A, to a feedback resistor network Rfb and R16 to minimize nonlinear drain to source resistance of U2A. As mentioned previously, using an amplifier or buffer amplifier such as U1A lowers the harmonic distortion caused by the drain to source resistance of U2A. In some examples when the enhancement mode FET (e.g., U2A) includes an SD5000 device, distortion was dramatically reduced by using a voltage follower amplifier to drive the feedback resistor Rfb compared to when the feedback resistor R7 was connected to the drain of U2A. The phase at Vout referenced to Vin is shifted in the following manner: 180 degrees?2 arctan(f/fc). Where f=frequency from the Vin signal source, and fc=1/[2π(C14×RdsU2A∥R19)]. Where RdsU2A is the drain to source resistance of FET U2A, and where RdsU2A∥R19=[(RdsU2A×R19)/(RdsU2A+R19)]. In some circuits R19 may be omitted such that R19=infinite ohms.
