1. Field of the Invention
The present invention relates to a radio frequency (RF) application circuit. More particularly, the present invention relates to a RF application circuit having a switch-block composed of a pair of bipolar junction transistors (BJTs) and applied in a capacitance unit of a LC resonance circuit.
2. Description of Related Art
Conventionally, in the LC (inductance-capacitance) filter or the LC oscillator of a radio frequency (RF) circuit, the switch used in a capacitance bank of a LC resonance circuit is usually implemented with N-channel metal-oxide-semiconductor (NMOS) transistor. This is because the gate side and drain side of the NMOS transistor are separated from each other, thus the DC level of the NMOS transistor is not affected when the NMOS transistor is turned on/off.
However, when the NMOS transistor is served as the switch in the capacitance bank of the LC resonance circuit, the NMOS transistor will be caused the RF circuit consuming a lot of power consumption, this is because the NMOS transistor with a high turned-on resistance value. In order to solve aforementioned problem, conventionally, a plurality of NMOS transistors, which are connected in parallel, can be reduced thereof turned-on resistance value. It is obviously, if the switch in the capacitance bank of the LC resonance circuit are utilized a plurality of NMOS transistors connected in parallel, the element size of the switch is increased, and the turned-off parasitic capacitance value thereof is also increased simultaneously.
In accordance with described above, the increased turned-off parasitic capacitance value of the switch further will be decreased the resolution of the capacitance bank in the LC resonance circuit, and thus the performance of the RF circuit thereof is adversely affected.