CH16Oscilators

Chapter 16Oscillators

Objectives

Describe the basic concept of an oscillator

Discuss the basic principles of operation of an oscillator

Describe the operation of the basic relaxation oscillator circuits

Analyze the operation of RC and LC feedback oscillators

Discuss the use of a 555 timer in an oscillator circuit

Introduction

Oscillators are circuits that produce a continuous signal of some type without the need of an input. These signals serve a variety of purposes. Communications systems, digital systems (including computers), and test equipment make use of oscillators.

The OscillatorAn oscillator is a circuit that produces a repetitive signal from a dc voltage.

The feedback oscillator relies on a positive feedback of the output to maintain the oscillations.

The relaxation oscillator makes use of an RC timing circuit to generate a nonsinusoidal signal such as square wave.

Figure 16–2 Basic elements of a feedback oscillator.

Thomas L. Floyd Electronic Devices, 7e

Feedback Oscillator PrinciplesThe feedback oscillator is widely used for generation of sine wave signals. The positive (in phase) feedback arrangement maintains the oscillations. The feedback gain must be kept to unity to keep the output from distorting.

Figure 16–3 Positive feedback produces oscillation.

Figure 16–4 General conditions to sustain oscillation.

Figure 16–5 When oscillation starts at t0, the condition Acl > 1 causes the sinusoidal output voltage amplitude to build up to a desired level. Then Acl decreases to 1 and maintains the desired amplitude.

Oscillators With RC Feedback Circuits

RC feedback oscillators are generally limited to frequencies of 1 Mhz or less. The three types of RC oscillators we will discuss are the Wien-bridge, the phase-shift, and the twin-T.

Figure 16–6 A lead-lag circuit and its response curve.

Oscillators With RC Feedback CircuitsThe lead-lag circuit of a Wien-bridge oscillator reduces the input signal by 1/3 and yields a response curve as shown. The frequency of resonance can be determined by the formula below.

fr = 1/2RC

The lead-lag circuit is in the positive feedback loop of Wien-bridge oscillator. The voltage divider limits gain. The lead lag circuit is basically a band-pass with a narrow bandwidth (high Q).

Figure 16–7 The Wien-bridge oscillator schematic drawn in two different but equivalent ways.

Oscillators With RC Feedback CircuitsSince there is a loss of about 1/3 of the signal in the

positive feedback loop, the voltage-divider ratio must be adjusted such that a positive feedback loop gain of 1 is produced. This requires a closed-loop gain of 3. The ratio of R1 and R2 can be set to achieve this.

Figure 16–9 Conditions for start-up and sustained oscillations.

To start the oscillations an initial gain greater than 1 must be achieved. The back-to-back zener diode arrangement is one way of achieving this. When dc is first applied the zeners appear as opens. This allows the slight amount of positive feedback from turn on noise to pass.

The lead-lag circuit narrows the feedback to allow just the desired frequency of these turn transients to pass. The higher gain allows reinforcement until the breakover voltage for the zeners is reached.

Oscillators With RC Feedback CircuitsAutomatic gain control is necessary to maintain a gain of

exact unity. The zener arrangement for gain control is simple but produces distortion because of the nonlinearity of zener diodes. A JFET in the negative feedback loop can be used to precisely control the gain. After the initial startup and the output signal increases the JFET is biased such that the negative feedback keeps the gain at precisely 1.

Figure 16–12

Oscillators With RC Feedback CircuitsAutomatic gain control is necessary to maintain a gain of

exact unity. The zener arrangement for gain control is simple but produces distortion because of the nonlinearity of zener diodes. A JFET in the negative feedback loop can be used to precisely control the gain. After the initial startup and the output signal increases the JFET is biased such that the negative feedback keeps the gain at precisely 1.

Oscillators With RC Feedback CircuitsThe phase shift oscillator utilizes three RC circuits to

provide 180º phase shift that when coupled with the 180º of the op-amp itself provides the necessary feedback to sustain oscillations. The gain must be at least 29 to maintain the oscillations. The frequency of resonance for the this type is similar to any RC circuit oscillator.

fr = 1/26RC

Oscillators With RC Feedback CircuitsThe twin-T utilizes a band-stop arrangement of RC circuits to block all but the frequency of operation in the negative feedback loop. The only frequency allowed to effectively oscillate is the frequency of resonance.

Figure 16–14

Oscillators With LC Feedback Circuits

For frequencies above 1 Mhz, LC feedback oscillators are used. We will discuss the Colpitts, Clapp, Hartley, Armstrong, and crystal-controlled oscillators. Transistors are used as the active device in these types.

The Colpitts oscillator utilizes a tank circuit (LC) in the feedback loop. The resonant frequency can be determined by the formula below. Since the input impedance affects the Q, an FET is a better choice for the active device.

fr = 1/2LCT

Figure 16–17 The attenuation of the tank circuit is the output of the tank (Vf) divided by the input to the tank (Vout).

Figure 16–18 Zin of the amplifier loads the feedback circuit and lowers its Q, thus lowering the resonant frequency.

Figure 16–19 A basic FET Colpitts oscillator.

Figure 16–20 Oscillator loading.

Figure 16–21

The Clapp is similar to the Colpitts with exception to the additional capacitor in the tank circuit. The same formula applies as for the Colpitts.

The Hartley oscillator is similar to the Clapp and Colpitts. The tank circuit has two inductors and one capacitor. The calculation of the resonant frequency is the same.

Oscillators With LC Feedback CircuitsThe Armstrong uses transformer coupling in the feedback loop. For this reason the Armstrong is not as popular.

Oscillators With LC Feedback CircuitsThe crystal-controlled oscillator is the most stable

and accurate of all oscillators. A crystal has a natural frequency of resonance. Quartz material can be cut or shaped to have a certain frequency. We can better understand the use of a crystal in the operation of an oscillator by viewing its electrical equivalent.

Since crystal has natural resonant frequencies of 20 Mhz or less, generation of higher frequencies is attained by operating the crystal in what is called the overtone mode. Overtones are usually odd multiples of a crystal’s fundamental.

Figure 16–26 Basic crystal oscillators.

Figure 16–27 Basic triangular-wave oscillator.

Relaxation OscillatorsRelaxation oscillators make use of an RC timing and a device that changes states to generate a periodic waveform.

This triangular-wave oscillator makes use of a comparator and integrator to actually produce both a triangle wave and square wave.

Relaxation OscillatorsOutput levels are set by the ratio of R2 and R3 times the maximum output of the comparator. The frequency of output can be determined by the formula below.

fr = 1/4R1C(R2/R3)

Figure 16–30

Relaxation Oscillators

The voltage-controlled sawtooth oscillator’s frequency can be changed by a variable dc control voltage. One possible type uses a programmable unijunction transistor.

Relaxation OscillatorsThe forward voltage of the PUT (VF) determines the frequency of the output. The formula below shows the relationship.

f = VIN/RiC(1/Vp-VF)

Figure 16–32

Figure 16–33 Output of the circuit in Figure 16–32.

Relaxation OscillatorsA square wave relaxation oscillator uses the charging and discharging of the capacitor to cause the op-amp to switch states rapidly and produce a square wave. The RC time constant determines the frequency.

Figure 16–36 Internal diagram of a 555 integrated circuit timer. (IC pin numbers are in parentheses.)

The 555 Timer As An OscillatorThe 555 timer is an integrated circuit that can be used in many applications. We will discuss its operation as a square wave oscillator. The frequency of output is determined by the external components R1, R2, and C. The formula below shows the relationship.

fr = 1.44/(R1 + 2R2)C Detailed operation is described within the text.

Figure 16–38 Operation of the 555 timer in the astable mode.

The 555 Timer As An Oscillator

Duty cycles can be adjusted by values of R1 and R2. The duty cycle is limited to 50% with this arrangement. To have duty cycles less than 50%, a diode is placed across R2. The two formulas show the relationship. (see following slide)

Duty Cycle >50 % = R1 + R2/R1 + 2R2 x 100%

Duty Cycle <50 % w/diode = R1/R1 + R2 x 100%

Figure 16–39 Frequency of oscillation (free-running frequency) of a 555 timer in the astable mode as a function of Cext and R1 + 2R2. The sloped lines are values of R1 + 2R2.

The 555 Timer As An Oscillator

Figure 16–41

The 555 Timer As An OscillatorThe 555 timer by be operated as a VCO with a control voltage applied to the CONT input (pin 5).

Figure 16–43 The VCO output frequency varies inversely with VCONT because the charging and discharging time of Cext is directly dependent on the control voltage.

Summary Sinusoidal oscillators operate with positive feedback. Two conditions for oscillation are 0º feedback phase shift and feedback loop gain of 1.

The initial startup requires the gain to be momentarily greater than 1.

RC oscillators include the Wien-bridge, phase shift, and twin-T. LC oscillators include the Colpitts, Clapp, Hartley, Armstrong, and crystal.

The crystal actually uses a crystal as the LC tank circuit and is very stable and accurate.

A voltage controlled oscillator’s (VCO) frequency is controlled by a dc control voltage.

A 555 timer is a versatile integrated circuit that can be used as a square wave oscillator or pulse generator.

Summary

Figure 16–44 Front panel of the function generator.

Figure 16–45 Block diagram of the function generator.

Figure 16–46 Schematic of the function generator.

Figure 16–47 The function generator circuit boards.

Figure 16–48 Results of tests on four faulty units. The scope screen shows the output voltage in each case.

Figure 16–49

Figure 16–50 Multisim file circuits are identified with a CD logo and are in the Problems folder on your CD-ROM. Filenames correspond to figure numbers (e.g., F16-50).

Figure 16–51

Figure 16–52

Figure 16–53

Figure 16–54

Figure 16–55

Figure 16–56

CH16Oscilators

floyd electronic

upper saddle

negative feedback

output signal

lc feedback

negative feedback

feedback loop

square wave

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