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Electronic Circuits

OSCILLATORS

Electronic Circuits

- The term oscillator naturally implies an oscillating or revolving motion.
- One example of mechanical oscillation is the pendulum in a grand farther clock, it oscillates back and forth.

Electronic Circuits

- Electronic oscillators operate in a similar manner, generating a continuously repetitive output signal that is sometimes used to time or synchronize operations.
- So what is an oscillator?
- An oscillator is a circuit that generates a repetitive AC signal.

Electronic Circuits

- The 60 hertz AC signal from the wall outlet is produced by an AC generator or alternator (oscillator) at the power station than transmitted to your home over the power lines.

Electronic Circuits

- The electronic oscillator uses no moving parts and is capable of producing AC signals ranging from a few hertz to many millions of hertz.
- It operates from the DC power supply and generates an AC signal.

Electronic Circuits

- A good oscillator circuit should have a uniform output.
- Uniform means not varying in frequency or amplitude all cycles should be identical.
- In order to produce uniform output, the circuit requires regenerative (positive) feedback.

Electronic Circuits

- OSCILLATORS ARE DIVIDED INTO THREE CATAGORIES:
- LC OSCILLATORS
- RC OSCILLATORS
- CRYSTAL OSCILLATORS

Electronic Circuits

- A simple oscillator circuit is the tank circuit.
- It consists of a capacitor and an inductor.

Electronic Circuits

- A capacitor and inductor directly connected together form something called a tank circuit, which oscillates (or resonates) at one particular frequency.
- At that frequency, energy is alternately shuffled between the capacitor and the inductor in the form of alternating voltage and current 90 degrees out of phase with each other.

Electronic Circuits

- When the power supply frequency for an AC circuit exactly matches that circuit's natural oscillation frequency as set by the L and C components, a condition of resonance will have been reached.

Electronic Circuits

- The problem with a LC tank is that the oscillating frequency becomes damped, the amplitude will eventually decrease to nothing.
- This is why we need positive feedback, to keep the circuit oscillating.

Electronic Circuits

- For the circuit to oscillate the feedback signal must be in phase with the input signal.
- By adding an OP amp to the circuit with regenerative feedback we can maintain a constant frequency oscillation.
- The basic oscillator requires an amplifier to replace circuit losses, frequency determining components and positive feedback to sustain oscillation.

Electronic Circuits

A BRIEF REVIEW OF OSCILLATORS:

- Needs an amp to replace circuit losses.
- Frequency determining components are necessary to set the frequency of oscillation.
- Positive feedback supplies a regenerative feedback to sustain oscillation.
- The oscillator must be self starting.

Electronic Circuits

WHICH ONE IS THE RC OSCILLATOR AND WHICH ONE IS THE LC OSCILLATOR?

Electronic Circuits

- Crystal materials produce piezoelectricity and crystals have a natural frequency of vibration.
- When pressure is applied to both sides of a crystal a potential of difference is developed.

CRYSTALL OSCILLATOR

Electronic Circuits

- The three most popular crystal materials are;
- Rochelle salt: Has the weakest crystal structure
- Tourmaline: Has the least electrical activity
- Quartz: Most commonly used

Electronic Circuits

- When the crystal is placed between two metal plates the circuit is modified and appears as a capacitor (Cp), and is represented as a parallel resonant circuit pictured below.

Electronic Circuits

- There are four different types of oscillator circuits;
- Hartley Crystal Oscillator: LC circuit, shunt fed, series connected, no crystal.
- Colpitts Crystal Oscillator: LC circuit, series connected with a crystal.
- Pierce Oscillator: No LC tank circuit, crystal connected.
- Butler Oscillator: Tuned LC circuit with a two transistor oscillator.

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