OSCILLATORS

An oscillator is a circuit that converts electrical energy in the form of DC to electrical energy in the form of AC at certain frequency and amplitude without any external input signal.

The output voltage can be either sinusoidal or non-sinusoidal, depending on the type of oscillator.

There are 2 types of oscillator:

1. Sinusoidal Oscillator - produces a sine-wave output signal.

2. Non-sinusoidal Oscillator produces complex waveforms such as square, rectangular, trigger, sawtooth, or trapezoidal.

1.0 BASIC OSCILLATOR BLOCK DIAGRAM

OscillatorDC supply voltageAC output voltage

Sinusoidal Oscillator(Harmonic Oscillator)

Non-sinusoidal Oscillator(Relaxation Oscillator)

Common AC signals

2.0 A BLOCK DIAGRAM OF OSCILLATOR

VfVoutAmplifier, Av

Feedback circuit, Attenuation+Phase shiftTiming Circuit

A basic oscillator consists of:1. AMPLIFIER with 'Active device' either Transistor or Op Amp.

An oscillator must provide amplification to input signal or voltage gain.

2. FEEDBACK CIRCUIT with passive components such as R-C or L-C combinations.

Produces phase shift and attenuation.

3. TIMING CIRCUIT

Generate audio frequency : RC network oscillator Generate radio frequency : LC network oscillator (Tank circuit)2.1 POSITIVE FEEDBACK

To start the oscillation with the constant amplitude, the oscillator needs a positive feedback circuit.

Positive Feedback: A part of output voltage of an amplifier is fed back to input through feedback circuit.

The in-phase feedback voltage, Vf is amplified to produce the output voltage.

A loop is created in which the signal sustains itself and a continuous sinusoidal output is produced.

To start the oscillations, output signal must be fed back in proper magnitude and phase.

VfIn-phase

2.2 REQUIREMENTS FOR OSCILLATION:

Oscillator circuit must satisfy the following 2 conditions known as BARKHAUSEN CONDITIONS:

1. The phase shift around the feedback loop must be 360 or 0.

This means, the phase shift through the amplifier and feedback network has to be 360 or 0.

2. The voltage gain, Acl around the closed feedback loop (loop gain) must unity.

The voltage gain for the closed feedback loop (Acl) is the product of the amplifier gain (AV) and the attenuation () of the feedback circuit.

Example:

If the amplifier has a gain of 100, the feedback circuit must have an attenuation of 0.01 to make the loop gain equal to 1.

Closed Feedback Loop Gain, Acl

i) Av = 1

Vout0.01Av100Vf

Inthissituation,the oscillator willoperate properly or the output signal will alwaysoscillate.

ii) Av < 1

Vout0.001Av100Vf

The oscillations die out after a few cycles. It is because the feedback voltage, Vf is not enough to maintain the output voltage.

iii) Av > 1

Vout0.1Av100Vf

Inthis case,the output voltagewill be largeruntil it reachesastate of saturation temperature and cutoff clipping.

2.3 START UP CONDITIONS

The requirements for the oscillation to start when DC supply voltage is FIRST turned ON.

Every resistor has certain free electrons. At room temperature these free electrons move randomly and generate a noise voltage across the resistor due to collisions. This voltage is also known as THERMAL NOISE VOLTAGE.

The resistor acts as small AC voltage source. When power supply is turned ON, for the first time this small AC noise voltage gets amplified and appears at the output terminal.

This amplified output is applied to feedback circuit and output of feedback circuit is fed back to the amplifier as an input.

This process is repeated and at one particular frequency, circuit satisfies the necessary conditions to start oscillation.

Amplifier, AvFeedback circuitAttenuation, +Phase shiftVoutVcc(to)

When oscillation starts, the condition Acl > 1 causes the sinusoidal output voltage amplitude to build up to a desired value.

Then, gain, Acl decreases to 1 and maintains the desired amplitude.

3.0 A SINUSOIDAL OSCILLATOR

A sinusoidal oscillator produces a sine-wave output signal.

The output signal is of constant amplitude with no variation in frequency.

Sinusoidal oscillator produces signals ranging from low audio frequencies to ultrahigh radio and microwave frequencies.

3.1 Types of Sinusoidal Oscillator:

3.1.1 RC oscillator

It uses a resistance-Capacitance network to determine the oscillator frequency.

They are suitable for low and moderate AUDIO frequency applications (5Hz to 1MHz).

Example: Phase-Shift Oscillator, Wein-Bridge Oscillator, Twin-T Oscillator.

3.1.2 LC oscillator

Inductors and capacitors are used either in series or parallel to determine the frequency.

LC oscillators, which use tank circuits, are commonly used for the higher RADIO frequencies (1 to 500 MHz).

Example: Colpitts, Clapp, Hartley, Armstrong, Crystal Ocsillators.

4.0 RC OSCILLATOR

4.1 PHASE SHIFT OSCILLATOR

A transistor in CE configuration acts as an amplifier or Op Amp as an inverting amplifier. It amplifies the input signal and produces a 180 voltage phase shift.

The feedback network introduces another 180 voltage shift at a resonant frequency (fr).

Total phase shift = Amplifier + Feedback circuit = 180 + 180 = 360 @ 04.1.1 RC FEEDBACK PHASE SHIFT NETWORK

* Example: 60 phase shift for each RC network.

The phase shift for each RC circuit is not equal. Each RC network can provide a maximum phase shift approaching 90 (ideal RC network).

However, the total phase shift for feedback circuit must equal 180.

This results of 360 voltage phase shift, which is the same as a 0 shift. 4.1.2 TRANSISTOR PHASE SHIFT OSCILLATOR

4.1.3 OP AMP PHASE SHIFT OSCILLATOR

Block diagram of the Phase Shift Oscillator with RC Feedback

Phase-shift oscillators are seldom used because they are extremely unstable (in terms of maintaining a constant frequency and amplitude).

The oscillation frequency (fr) of this Oscillator is:

Where:fr = Output frequency (Hertz)R = Resistance ()C = Capacitance (Farad)N = The number of RC stages.

If resistance R = R1 = R2 = R3 and capacitance C = C1 = C2 = C3, there are 3 RC stages (N=3).

The attenuation, of the three section RC network:

The closed loop voltage gain of the op amp: Example:

A) Determine the value of Rf necessary for the circuit to operates as an oscillator.B) Determine the frequency of oscillation.

Given: C = C1 = C2 = C3 = 0.001F and R = R1 = R2 = R3 = 10k.

R1 R2 R3C1 C2 C3

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