An electronic device that generates sinusoidal oscillations of desired frequency is known as sinusoidal oscillator. It receives DC energy and changes it into AC energy of desired frequency. In oscillators, the feedback voltage is in phase with the input voltage.
• For sustained oscillations/un-damped oscillations: Av * B = 1 (Barkhausen Criterion)
• Every resistor contains some free electrons. Because of ambient temperature, these free electrons move randomly in different directions and generate a noise voltage across the resistor. These noise voltages generated by the resistors act as input voltage for an oscillator.

#### Types:

At frequency under 1 MHz, We can use RC oscillators to produce almost perfect sine waves and above 1 MHz, LC oscillators are used.

1. RC oscillators: Wien Bridge, Twin-T and Phase Shift.
2. LC oscillators: Colpitts, Armstrong, Hartley, Clapp and Crystal.

#### Wien Bridge Oscillator:

Phase response curve of lead-lag circuit shown at Very Low Frequencies, the series capacitor appears open to input signal and the phase angle is positive. At Very High Frequencies, the shunt capacitor looks shorted and the phase angle is negative.
• It has a frequency range of 5 MHz to 1 MHz.
• Tungsten lamp increase with voltage.
• It uses both positive and negative feedback.
• Used in commercial audio series.
• Also used as Signal Generator in laboratories.

#### Phase Shift Oscillator:

In this oscillator, Three RC phase shift circuit provides 180° phase shift (60° each) and an Amplifier provides additional 180° because of inverting input. As a result a total 360° phase shift is produced.
• fr = 1/(2*π*R*C*√6)

#### Colpitts Oscillators:

A 180° phase shift is produced by capacitor voltage divider and other by transistor amplifier.
• Use RF oscillators.

#### Armstrong Oscillator:

It uses transformer coupling for the feedback signal. We often used term ticker coil because it feedback the signal that sustains the oscillations.
• fr = 1 / ( 2*π*√(L*C) ), B = M/L and Av(min) = L/M
• Where: M is Mutual Inductance and L is Self Inductance.
• Not used because of bulky size.

#### Hartley Oscillator:

It uses inductive voltage divider for feedback.
• B = L2/L1 and Av(min) = L1/L2
• L = L1 + L2 + 2M and fr = 1 / ( 2*π*√(L*C) )

#### Clapp Oscillator:

It is the enhanced version of colpitts oscillator. Feedback is provided by capacitive voltage divider but an extra capacitor is used in series with inductor.
• C = 1 / (1/C1 + 1/C2 + 1/C3), C3<<<<C1,C2
• So C ≅ C3
• fr ≅ 1 / ( 2*π*√(L*C3)
• In colpitts oscillator C1 and C2 are shunted by transistor and stray capacitance which alters the value of C1 and C2 slightly. Therefore the fr was depended on transistor and stray capacitance.
• But in clapp oscillator C3 is independent So fr is more stable and accurate. That is why mostly used.

#### Crystal Oscillator:

For most accurate and stable oscillations we use a crystal oscillator.

#### Reference:

Albert Malvino and David J Bates, “Electronic Principles”, 7th Edition, TATA McGRAW HILL.