In transceivers, transmitter and receiver detuning (sometimes termed de-Q-ing) can be required to prevent transmitter radiation from entering and desensitizing receiver circuitry and saturating the receiver preamplifier. In other words, during a transmit mode of operation of the transceiver, the receiver should be detuned so that signals from the transmitter are not received (or received at much lower power levels), and during a receive mode of operation, the transmitter should be detuned. Diodes are routinely used as bi-stable elements for detuning that are turned on or off by providing DC bias on the diodes. A conventional detuning circuit is shown in FIG. 1.
In the example of FIG. 1, the detuning circuit includes a pair of oppositely-oriented diodes 102, 104 connected to opposite ends of one side of a transformer 110. Receiver circuitry operates at very low voltages, and therefore for receiver detuning an external command voltage is applied to bias the diodes 102, 104. Accordingly, the detuning circuit further includes a power source 112, such as a battery, and switch 114 that selectively connects the power source 112 to the diodes 102, 104. The power source 112 is connected to the center tap of the transformer 110. For receiver detuning, the switch 114 is closed to actively bias the diodes 102, 104. The large current and voltages carried in the antenna 120 necessitate introduction of the transformer 110 to reduce power-handling requirements on the diodes 102, 104. Still, large bias voltages are required, making the detuning complicated. In addition, resistors 106, 108 are connected in series with the diodes 102, 104 to limit the DC current through the diodes 102, 104 when the switch 114 is closed. These resistors 106, 108 need to be relatively large (high resistive values) to avoid burn-out of the diodes, which is a disadvantage. In addition, the large parasitic capacitance of the diodes 102, 104 limits their usefulness.