Most of our studies in previous chapters have focused on the analysis and design of amplifiers. In this chapter, we turn our attention to another important class of analog circuits, namely, oscillators. From your laptop computer to your cell phone, today’s electronic devices use oscillators for numerous purposes, and pose interesting challenges. For example, the clock driving a 3 GHz microprocessor is generated by an on-chip oscillator running at 3 GHz. Also, a WiFi transceiver employs a 2.4 GHz or 5 GHz on-chip oscillator to generate a “carrier.” Shown below is the outline of the chapter. The reader is encouraged to review Chapter 12 before delving into oscillators.
13.1 GENERAL CONSIDERATIONS
We know from previous chapters that an amplifier senses a signal and reproduces it at the output, perhaps with some gain. An oscillator, on the other hand, generates a signal, typically a periodic one. For example, the clock in a microprocessor resembles a square wave (Fig. 13.1).
How can a circuit generate a periodic output without an input? Let us return to our study of amplifier stability in Chapter 12 and recall that a negative-feedback circuit can oscillate if Barkhausen’s criteria are met. That is, as shown in Fig. 13.2, we have