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Here is a summary of the material covered in this chapter:
Oscillators are circuits that produce an output waveform without an external signal source. The key to oscillator operation is positive feedback. A positive feedback network produces a feedback voltage ( Figure 18-1 also illustrates the basic principle of how the oscillator produces an output waveform without any input signal. In Figure 18-1a, the switch is momentarily closed, applying an input signal to the circuit. This results in a signal at the output from the amplifier, a portion of which is fed back to the input by the feedback network. In Figure 18-1b, the switch is now open, but the circuit continues to oscillate because the feedback network is supplying the input to the amplifier. The feedback network delivers an input to the amplifier, which in turn generates an input for the feedback network. This circuit action is referred to as regenerative feedback and is the basis for all oscillators. An oscillator needs a brief trigger signal to start the oscillations. Most oscillators provide their own trigger simply by turning the circuit on. This principle is explained in detail at the end of Section 18.2 of the text. So far, we have established two requirements for oscillator operation:
![]() FIGURE 18-1 Regenerative feedback.
There is one other requirement for oscillator operation. The circuit must fulfill a condition referred to as the Barkhausen criterion. This relationship is called the Barkhausen criterion. If this criterion is not met, one of the following occurs:
These principles are illustrated in Figure 18-2. ![]() FIGURE 18-2 The effects of ![]() If
Phase-Shift Oscillators The phase-shift oscillator contains three RC circuits in its feedback circuit. In your study of basic electronics, you learned that an RC circuit produces a phase shift at a given frequency that can be calculated using: where The phase angle relationship shows that
![]() FIGURE 18-3 Basic phase-shift oscillator. The three RC networks in the phase-shift oscillator in Figure 18-3 produce a combined phase shift of 180° at a resonant frequency ( You would think that each RC network in the phase shift oscillator would produce a 60° phase shift (3 × 60° = 180°), but this is not true. Each RC circuit loads the previous RC circuit, so their phase shifts are not equal. However, the total phase shift must equal 180° for the circuit to operate as an oscillator. In reality, phase-shift oscillators are seldom used because they are extremely unstable (in terms of maintaining a constant frequency and amplitude). It is introduced here simply to help us understand the basic principles of how oscillators work, and because it is easy to build a phase-shift oscillator by mistake. A multistage amplifier can oscillate under certain conditions. Refer to Figure 18.5 of the text.
The Wien-Bridge Oscillator The Wien-bridge oscillator is a commonly used low-frequency oscillator. This circuit achieves regenerative feedback by introducing no phase shift (0°) in the positive feedback path. As shown in Figure 18-4, there are two RC circuits in the positive feedback path (output to noninverting input).
![]() FIGURE 18-4 Wien-bridge oscillator.
The The negative feedback path is from the output to the inverting input of the op-amp. Note the differences from a normal negative feedback circuit. Two diodes have been added in parallel with Earlier we said that the Wien-bridge oscillator is a common low-frequency oscillator. As frequency increases, the propagation delay of the op-amp can begin to introduce a phase shift, which causes the circuit to stop oscillating. Propagation delay is the time required for the signal to pass through a component (in this case the op-amp). Most Wien-bridge oscillators are limited to frequencies below 1 MHz. Refer to Figure 18.9 of the text for a summary of Wien-bridge oscillator characteristics.
The Colpitts Oscillator The Colpitts oscillator is a discrete LC oscillator that uses a pair of tapped capacitors and an inductor to produce regenerative feedback. A Colpitts oscillator is shown in Figure 18-5. The operating frequency is determined by the tank circuit. By formula:
![]() FIGURE 18-5 Colpitts oscillator. The key to understanding this circuit is knowing how the feedback circuit produces its 180° phase shift (the other 180° is from the inverting action of the CE amplifier). The feedback circuit produces a 180° voltage phase shift as follows:
The first two points are fairly easy to see.
![]() FIGURE 18-6 Figure 18-6 is the equivalent representation of the tank circuit in the Colpitts oscillator. Lets assume that the inductor is the voltage source and it induces a current in the circuit. With the polarity shown across the inductor, the current causes potentials to be developed across the capacitors with the polarities shown in the figure. Note that the capacitor voltages are 180° out of phase with each other. When the polarity of the inductor voltage reverses, the current reverses, as does the resulting polarity of the voltage across each capacitor (keeping the capacitor voltages 180° out of phase). The value of the feedback voltage is determined (in part) by the
The validity of these equations is demonstrated in Example 18.1 of the text. As with any oscillator, the product of As with any tank circuit, this one will be affected by a load. To avoid loading effects (the circuit loses some efficiency), the output from a Colpitts oscillator is usually transformer-coupled to the load, as shown in Figure 18.14 of the text. Capacitive coupling is also acceptable so long as: ![]() where
Other LC Oscillators Three other oscillators are mentioned briefly in this chapter: the Hartley oscillator, the Clapp oscillator, and the Armstrong oscillator. The Hartley oscillator is similar to the Colpitts except that it uses a pair of tapped coils instead of two tapped capacitors. For the circuit in Figure 18-7, the output voltage is developed across The tank circuit, just like in the Colpitts, determines the operating frequency of the Hartley oscillator. As the tapped inductors are in series, the sum of
![]() FIGURE 18-7 Hartley oscillator.
The Clapp oscillator is simply a Colpitts oscillator with an extra capacitor in series with the coil. It is labeled as
![]() FIGURE 18-8 Clapp oscillator. The Armstrong oscillator uses a transformer to achieve the 180°
phase shift required for oscillation, as shown in Figure 18-9. As you can see from the figure, the output from the transistor is applied to the primary of the transformer and the feedback is taken from the secondary. Note from the polarity dots on the transformer that the secondary is inverted relative to the primary. This is how the 180°
phase shift is accomplished. The capacitor,
![]() FIGURE 18-9 Armstrong oscillator. Note: Any of the LC oscillators in this section can be constructed using FETs or op-amps. There are also several two-stage oscillator circuits. Many of these circuits can be constructed in common-base or common-gate configuration. To identify any LC oscillator, look for the circuit recognition features. These recognition features are listed in Table 18-1. TABLE 18.1
Crystal-Controlled Oscillators In applications where extremely stable operating frequencies are required, the oscillators that we have studied so far come up short. They can experience variations in both frequency and amplitude for several reasons:
In any system where stability is paramount, crystal-controlled oscillators are used. Crystal-controlled oscillators use a quartz crystal to control the operating frequency. The key to the operation of a crystal-controlled oscillator is the piezoelectric effect, which means that the crystal vibrates at a constant rate when it is exposed to an electric field. The physical dimensions of the crystal determine the frequency of vibration. Thus, by cutting the crystal to specific dimensions, we can produce crystals that have very exact frequency ratings. There are three commonly used crystals that exhibit piezoelectric properties. They are Rochelle salt, quartz, and tourmaline. Rochelle salt has the best piezoelectric properties but is very fragile. Tourmaline is very tough, but its vibration rate is not as stable. Quartz crystals fall between the two extremes and are the most commonly used. Quartz crystals are made from silicon dioxide (
![]() FIGURE 18-10 Crystal symbol, equivalent circuit, and frequency response. The electrical operation of the crystal is a function of its physical properties, but it can still be represented by an equivalent circuit. The equivalent circuit in Figure 18-10 represents specific crystal characteristics:
L = the inductance of the crystal R = the resistance of the crystal The response curve in Figure 18-10 is explained in detail in Section 18.6 of the text. The primary points are as follows:
This means that a crystal can be used to replace either a series or a parallel resonant LC circuit. It should also be noted that there is very little difference between A crystal can produce outputs at its resonant frequency and at harmonics of that resonant frequency. This concept was introduced when we looked at tuned class C amplifiers. The resonant frequency is often referred to as the fundamental frequency, and the harmonic frequencies as overtones. Crystals are limited by their physical dimensions to frequencies of 10 MHz or below. If the circuit is tuned to one of the harmonic frequencies of the crystal (overtones), then we can produce stable outputs much higher than the 10 MHz limit of the crystal itself. This type of circuit is said to be operating in overtone mode. A Colpitts oscillator can be modified into a crystal-controlled oscillator (CCO) as shown in Figure 18-11. Note that the crystal is in series with the feedback path and is operating in series-resonant mode ( ![]() FIGURE 18-11 Crystal-controlled Colpitts oscillator. Oscillator Troubleshooting Oscillators can be very challenging to troubleshoot. With the exception of the biasing resistors, every component is directly involved in producing an output signal. If any of these components fails, then there will be no output at all. As with any circuit, you start troubleshooting an oscillator by eliminating the obviouscheck the power supply. If the oscillator is faulty, then you should keep in mind the following:
In many cases it is simpler and less time consuming to simply replace the reactive components. If you decide to pursue this route, replace one component at a time and retest the circuits operation. When it starts to operate properly, you know you have found the faulty component.
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