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The Oscillator

A circuit that produces a repeating waveform with only DC as an input is called an

oscillator.

Oscillators are used in many electronic systems, such as radio, TV, telephones, and

industrial systems. An oscillator can produce many types of output waveforms such as sine waves, triangle

waves, or square waves.

Oscillator Principles

Oscillators operate with positive feedback.

The output of an oscillator is fed back to the input in-phase through some type of

feedback network.

The in-phase feedback is called positive feedback.

The active element in an oscillator is an amplifier. The amplifier may be of several

varieties, such as BJT, FET, or op-amp.

The voltage gain of the amplifier must be enough to overcome the loss in the feedback

network.

To start oscillation, it is necessary to have a voltage gain greater than 1 so that the

waveform output will build up in amplitude.

The voltage gain must be reduced to about unity to sustain the oscillations after they

have started, and the phase shift around the feedback loop must be 0.

Oscillators with RC Feedback Circuits

The phase-shifting feedback network of an oscillator can consist of an RC network.

A very common RC feedback oscillator is the Wien-bridge oscillator.

A Wien-bridge oscillator uses a lead-lag network in the feedback loop. A lead-lag network changes the phase relationship from a leading to a lagging phase

angle as the frequency changes.

0 phase shift is achieved at a frequency called the resonant frequency.

In a Wien-bridge oscillator, only the desired resonant frequency is fed back in-phase. The

output is a sine wave at the resonant frequency fr. fr = 1.2RC where R = R1 = R2 and C =C1 = C2.

Wien-bridge oscillators often use back-to-back zener diodes in parallel with the feedback

resistor in the negative loop. These diodes increase the gain required during the start-upphase of the oscillator.

A phase-shift oscillatoruses a three-section RC network to provide the necessary in-

phase feedback.

The resistors and capacitors in the RC network usually have the same value, R and C.The frequency of oscillation is found by

A twin-T oscillatoruses two RC networks. One is a T-type low-pass filter and the other

is a T-type high-pass filter. These filters are in the negative feedback loop. These filtersact as a band-reject filter. The only frequency where negative feedback is minimum is theresonant frequency.

Most oscillators that use RC feedback networks are usable at frequencies up to about 1

MHz.

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Oscillators with LC Feedback Circuits

High frequency oscillators are frequently of the LC feedback type.

LC oscillators use a parallel resonant or tank circuit to establish the resonant

frequency.

The phase shift across a tank circuit is 180.

A Colpitts oscillator uses a tank circuit with two capacitors in series. The feedback

connection is from the junction between the two capacitors.

The resonant frequency, fr, of a Colpitts oscillator is found by

,where C is the total series capacitance in the tank.

A load applied to the oscillator will act to change the resonant frequency by reducing the

Q.

A variation of the Colpitts is the Clapp oscillator.

The Clapp oscillator uses the two series capacitors, as does the Colpitts. In addition, it

uses a third capacitor in series with the inductor in the tank. The frequency is determined

largely by the value of this extra capacitor. An oscillator similar to the Colpitts is the Hartley oscillator. This circuit uses a split

inductor instead of the two capacitors.

The resonant frequency for the Hartley oscillator is found with the familiar formula

mentioned above.

The Armstrong oscillatoruses a feedback loop that is transformer-coupled from the

tank circuit back to the input.

A very stable and accurate oscillator is the crystal oscillator.

Crystal oscillators usually incorporate a quartz crystal. These crystals exhibit what is

called the piezoelectric effect.

This piezoelectric effect causes a slice of quartz crystal to vibrate at a natural frequency

when a mechanical force is applied to it. While vibrating, a voltage will be producedacross the crystal at its natural frequency.

Conversely, when an ac voltage is applied across the crystal, it will mechanically vibrate

at its natural frequency.

A crystal acts as a mechanical tank circuit. The frequency of oscillation is dependent

upon the size and shape of the crystal.

As in a tank circuit, a crystal in series with the feedback will provide minimum impedance

at fr. A crystal in parallel with the feedback loop will provide maximum impedance at the fr.

Nonsinusoidal Oscillators

A oscillator can produce triangle waves. A simple circuit uses a comparator followed by

an integrator.

A voltage-controlled oscillator (VCO) is an oscillator whose frequency can be changedby an applied voltage.

The 555 Timer as an Oscillator

A multivibrator is a circuit that produces square wave.

There are two general types of multivibrators, astable or free-running, and monostable

or one-shot. In addition, there is a bistable multivibrator used in digital circuits.

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An astable multivibrator produces a constant train of square waves as long as the circuit

is turned on.

A monostable multivibrator produces one output square wave for each input trigger.

The 555 timer consists of two comparators, a flip-flop, a discharge transistor, and a buffer

output all mounted in an IC chip.

To produce the astable state, the 555 timer uses an external capacitor and two resistors.

The frequency of oscillation is found by f = 1.44/(R1 + 2R2)Cext. The duty cycle of the timer in the astable state can be found by

Duty cycle = (R1 + R2)/(R1 + 2R2) 100%.

The capacitor charges through R1 and R2 and discharges through R2. A minimum duty

cycle of 50% can be achieved. A lower duty cycle requires a diode in parallel with R2.Under this condition,Duty cycle = R1/(R1 + R2) 100%.