VCO (Voltage-Controlled Oscillator)
There are many single-ended oscillators in RF circuit design. In this subsection, only the “three-point” type oscillators are discussed. Among the three-point type oscillators, the most popular one applied in communication systems is the Clapp oscillator, which will therefore be discussed in Section 20.1.3 in somewhat detail.
A three-point type oscillator contains only one device, either a bipolar transistor or a MOSFET (metal–oxide–semiconductor field-effect transistor). The AC equivalents of these three points are the base, collector, and emitter for a bipolar transistor, or the gate, drain, and source for a MOSFET. Figure 20.1 shows the AC equivalent of a three-point oscillator built by a MOSFET. The three nodes marked with 1, 2, and 3, are connected to the gate, drain, and source of the MOSFET, respectively. On the other hand, the three parts with impedances Z1, Z2, and Z3 are connected between nodes 1 and 3, 3 and 2, and 2 and 1, respectively.