As is known in the art, a radio frequency (RF) switch circuit (hereinafter switch or switch circuit) is a device used to connect and disconnect RF signal paths in an electrical circuit. When connecting RF signal paths, a switch generally provides a bi-directional RF signal path. Thus, signals fed to an input port of the switch appear at an output port of the switch and vice-versa.
When an RF switch provides an RF signal path having a relatively low insertion loss characteristic between two ports, the switch is generally referred to as being in the “ON” position. When the switch provides an RF signal path having a relatively high insertion loss characteristic between the common port and the branch port the switch is generally referred to as being in the “OFF” position.
The electrical characteristics of an RF switch include isolation, insertion loss, switching speed and RF power handling capability. Depending upon the particular application, it is often necessary to optimize one of these electrical characteristics by trading off the performance of the other electrical characteristics of the switch.
The isolation and insertion-loss characteristics of an RF switch having a single input port and a single output port are generally determined by providing an RF signal to the input port and measuring the difference between the power level of a signal provided at the switch output port with the switch OFF and with the switch ON, respectively.
Referring now to FIG. 1, an RF switch includes a transmission line T1 coupled between first and second switch ports P1, P2. A field effect transistor (FET1) acts as a switching element when placed in shunt with transmission line T1. When transistor FET1 is pinched off by applying a negative voltage on a gate terminal G1 beyond a pinch off voltage (typically about −3V), transistor FET1 is provided having a high impedance characteristic between source and drain electrodes S1, D1 (i.e. the transistor FET1 is in its “off” state) and thus an RF signal will propagate along transmission line T1 from port P1 to port P2.
When the bias voltage applied to gate electrode is zero volts or positive, transistor FET1 is provided having a low impedance characteristic between source and drain electrodes S1, D1. That is, the transistor FET1 is biased into its conductive state and thus is in its “on” state. Consequently, an RF signal injected to port P1 will be reflected back to port P1 and will not reach port P2. Thus, FIG. 1 illustrates one embodiment of a conventional switch having a FET shunt coupled to ground with respect to a transmission line connecting ports P1 and P2.
Referring now to FIG. 2, another embodiment of a conventional switch includes a FET inserted in series with a transmission line. When the FET is on (i.e. the FET is biased into its conductive or low impedance state), an injected signal at port P1 is coupled to port P2 through the low impedance signal path provided by the FET. When the FET is off (i.e. the FET is biased into its nonconductive or high impedance state), an injected signal at port P1 will be reflected and will not appear at port P2. Thus, FIG. 2, illustrates a second embodiment of a conventional switch where a switching FET is inserted in series with the transmission line connecting ports P1 and P2.
When an RF signal level is in the range of about 100 watts (W), it generates in the range of about 200 volts peak-to-peak (Vp-p) across a 50 ohm load or draws about 4 amperes peak-to-peak (Ap-p) through a 50 ohm load. Therefore, the first requirement for the FET switch element in the configurations illustrated in FIGS. 1 and 2 is that it must to be capable of handling 4 Ap-p. Secondly, the FET breakdown voltage must be large enough to sustain 200 Vp-p. The gate control voltage for the off state will have to be −100V or less to keep it pinched off when the RF signal swings to −100V. Maximum differential voltage between the drain and gate will reach 200V when the RF signal swings to +100V while the control voltage on the gate is maintained at −100V. It is thus necessary for the FET breakdown voltage to be larger than 200V. This is a difficult requirement for the device.