RF Circuit Design, 2nd Edition

Chapter 20

VCO (Voltage-Controlled Oscillator)

20.1 “Three-Point” Types of 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.

Figure 20.1 AC equivalent of a three-point type of oscillator. (a) Tank circuit in input side. (b) Tank circuit in output side. (c) Tank circuit in both of input and output side.

These three parts form a tank circuit loop. The AC equivalents plotted in Figure 20.1(a)–(c) are exactly the same circuit. The only difference between Figure 20.1(a), (b), ...