An oscillator is a circuit that generates a repetitive AC signal. The frequency of the AC signal may vary from a few hertz to many millions of hertz.
The oscillator is an alternative to the mechanical generator used to produce electrical power. The advantages of the oscillator are the absence of moving parts and the range over which the AC signal can be produced.
The output of an oscillator may be a sinusoidal, rectangular, or sawtooth waveform, depending on the type of oscillator used. The main requirement of an oscillator is that the output be uniform; that is, the output must not vary in either frequency or amplitude.
Circuit Diagrams & Operation of Oscillators
When an inductor and a capacitor are connected in parallel, they form what is called a tank circuit. When a tank circuit is excited by an external DC source, it oscillates; that is, it produces a back-and-forth current flow.
If it were not for the resistance of the circuit, the tank circuit would oscillate forever. However, the resistance of the tank circuit absorbs energy from the current, and the oscillations of the circuit are dampened. For the tank circuit to maintain its oscillation, the energy that is dissipated must be replaced. The energy that is replaced is referred to as positive feedback.
Positive feedback is the feeding back into the tank circuit of a portion of the output signal to sustain oscillation. The feedback must be in phase with the signal in the tank circuit.
Figure 1 shows a block diagram of an oscillator. The basic oscillator can be broken down into three sections. The frequency-determining oscillator circuit is usually an LC tank circuit. An amplifier increases the output signal of the tank circuit. A feedback circuit delivers the proper amount of energy to the tank circuit to sustain oscillation. The oscillator circuit is essentially a closed loop that uses DC power to maintain AC oscillations.
Sinusoidal Oscillators
Sinusoidal oscillators are oscillators that produce a sine-wave output. They are classified according to their frequency-determining components. The three basic types of sinusoidal oscillators are LC oscillators, crystal oscillators, and RC oscillators.
LC oscillators use a tank circuit of capacitors and inductors, connected either in series or parallel, to determine the frequency. Crystal oscillators are like LC oscillators except that crystal oscillators maintain a higher degree of stability.
LC and crystal oscillators are used in the radio frequency (RF) range. They are not suitable for low-frequency applications. For low-frequency applications, RC oscillators are used.
RC oscillators use a resistance-capacitance network to determine the oscillator frequency.
Three basic types of LC oscillator are the Hartley oscillator, the Colpitts oscillator, and the Clapp oscillator.
Figures 2 and 3 show the two basic types of Hartley oscillator. The tapped inductor in the tank circuit identifies these circuits as Hartley oscillators. The disadvantage of the series-fed Hartley (Figure 2) is that DC current flows through a portion of the tank circuit. The shunt-fed Hartley (Figure 3) overcomes the problem of DC current in the tank circuit by using a coupling capacitor in the feedback line.
The Colpitts oscillator (Figure 4) is like the shunt-fed Hartley except that two capacitors are substituted for the tapped inductor. The Colpitts is more stable than the Hartley and is more often used.
The Clapp oscillator (Figure 5) is a variation of the Colpitts oscillator. The main difference is the addition of a capacitor in series with the inductor in the tank circuit. The capacitor allows tuning of the oscillator frequency.
Temperature changes, aging of components, and varying load requirements cause oscillators to become unstable. When stability is a requirement, crystal oscillators are used.
Crystals are materials that can convert mechanical energy to electrical energy when pressure is applied or can convert electrical energy to mechanical energy when a voltage is applied. When an AC voltage is applied to a crystal, the crystal stretches and compresses, creating mechanical vibrations that correspond to the frequency of the AC signal.
Crystals, because of their structure, have a natural frequency of vibration. If the AC signal applied matches the natural frequency, the crystal vibrates more. If the AC signal is different from the crystal’s natural frequency, little vibration is produced. The crystal’s mechanical frequency of vibration is constant, making it ideal for oscillator circuits.
The most common materials used for crystals are Rochelle salt, tourmaline, and quartz. Rochelle salt has the most electrical activity, but it fractures easily. Tourmaline has the least electrical activity, but it is the strongest. Quartz is a compromise: It has good electrical activity and is strong.
Quartz is the most commonly used crystal in oscillator circuits. The crystal material is mounted between two metal plates, with pressure applied by a spring so that the metal plates make electrical contact with the crystal. The crystal is then placed in a metal package.
Figure 6 shows a shunt-fed Hartley oscillator with the addition of a crystal. The crystal is connected in series with the feedback circuit. If the frequency of the tank circuit drifts from the crystal frequency, the impedance of the crystal increases, reducing feedback to the tank circuit. This allows the tank circuit to return to crystal frequency.
Figure 7 shows a Colpitts oscillator connected the same way as the Hartley crystal oscillator. The crystal controls the feedback to the tank circuit. The LC tank circuit is tuned to the crystal frequency.
Figure 8 shows a Pierce oscillator. This circuit is similar to the Colpitts oscillator except that the tank-circuit inductor is replaced with a crystal. The crystal controls the tank-circuit impedance, which determines the feedback and stabilizes the oscillator.
Figure 9 shows a Butler oscillator. This is a two-transistor circuit. It uses a tank circuit, and the crystal determines the frequency. The tank circuit must be tuned to the crystal frequency or the oscillator does not work. The advantage of the Butler oscillator is that a small voltage exists across the crystal, reducing stress on the crystal.
By replacing the tank-circuit components, the oscillator can be tuned to operate on one of the crystal’s overtone frequencies. RC oscillators use resistance-capacitance networks to determine the oscillator frequency.
There are two basic types of RC oscillators that produce sinusoidal waveforms: the phase-shift oscillator and the Wien-bridge oscillator.
A phase-shift oscillator is a conventional amplifier with a phase-shifting RC feedback network (Figure 10). The feedback must shift the signal 180 degrees. Because the capacitance reactance changes with a change in frequency, it is the frequency-sensitive component.
Stability is improved by reducing the amount of phase shift across each RC network. However, there is a power loss across the combined RC network. The transistor must have enough gain to offset these losses.
A Wien-bridge oscillator is a two-stage amplifier with a lead-lag network and voltage divider (Figure 11). The lead-lag network consists of a series RC network (R1 C1) and a parallel network. It is called a lead-lag network because the output phase angle leads for some frequencies and lags for other frequencies.
At the resonant frequency, the phase shift is zero, and the output voltage is maximum. Resistors R3 and R4 form the voltage-divider network, which is used to develop the degenerative feedback. Regenerative feedback is applied to the base and degenerative feedback is applied to the emitter of oscillator transistor Q1.
The output of transistor Q1 is capacitively coupled to the base of transistor Q2 where it is amplified and shifted and required 180 degrees. The output is coupled by capacitor C3 to the bridge network.
Figure 12 shows an integrated circuit Wien-bridge oscillator. The inverting and non-inverting inputs of the op-amp are ideal for use as a Wien-bridge oscillator. The gain of the op-amp is high, which offsets the circuit losses.
Non-sinusoidal Oscillators
Nonsinusoidal oscillators are oscillators that do not produce a sine-wave output. There is no specific nonsinusoidal waveshape. The nonsinusoidal oscillator output may be a square, sawtooth, rectangular, or triangular waveform, or a combination of two waveforms. A common characteristic of all nonsinusoidal oscillators is that they are a form of relaxation oscillator.
A relaxation oscillator stores energy in a reactive component during one phase of the oscillation cycle and gradually releases the energy during the relaxation phase of the cycle. Blocking oscillators and multivibrators are relaxation oscillators.
Figure 13 shows a blocking oscillator circuit. The reason for the name is that the transistor is easily driven into the blocking (cut-off) mode. The blocking condition is determined by the discharge from capacitor C1. Capacitor C1 is charged through the emitter-base junction of transistor Q1. However, once capacitor C1 is charged, the only discharge path is through resistor R1.
The RC time constant of resistor R1 and capacitor C1 determines how long the transistor is blocked or cut off and also determines the oscillator frequency. A long time constant produces a low frequency; a short time constant produces a high frequency.
If the output is taken from an RC network in the emitter circuit of the transistor, the output is a sawtooth waveshape (Figure 14). The RC network determines the frequency of oscillation and produces the sawtooth output. Transistor Q1 is forward biased by resistor R1. As transistor Q1 conducts, capacitor C1 charges quickly. The positive potential on the top plate of capacitor C1 reverse biases the emitter junction, turning off transistor Q1. Capacitor C1 discharges through resistor R2, producing the trailing portion of the sawtooth output. When capacitor C1 discharges, transistor Q1 is again forward biased and conducts, repeating the action.
Capacitor C1 and resistor R2 determine the frequency of oscillation. By making resistor R2 variable, the frequency can be adjusted. If resistor R2 offers high resistance, a long RC time constant results, producing a low frequency of oscillation. If resistor R2 offers low resistance, a short RC time constant results, producing a high frequency of oscillation.
A multivibrator is a relaxation oscillator that can function in either of two temporarily stable conditions and is capable of rapidly switching from one temporary state to the other.
Figure 15 shows a basic free-running multivibrator circuit. It is basically an oscillator consisting of two stages coupled so that the input signal to each stage is taken from the output of the other. One stage conducts while the other stage is cut off, until a point is reached where the stages reverse their conditions.
The circuit is free-running because of regenerative feedback. The frequency of oscillation is determined by the coupling circuit.
An astable multivibrator is one type of free-running multivibrator. The output of an astable multivibrator is rectangular. By varying the RC time constants of the coupling circuits, rectangular pulses of any desired width can be obtained. By changing the values of the resistor and capacitor, the operating frequency can be changed. The frequency stability of the multivibrator is better than that of the typical blocking oscillator.
An integrated circuit that can be used as an astable multivibrator is the 555 timer Figure 16 shows a schematic diagram in which the 555 timer is used as an astable multivibrator. The output frequency is determined by resistors RA and RB and capacitor C1. This circuit finds wide application in industry.
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