Adama University EEng 2202 Electrical Engineering 1 4.1 Introduction In electronics periodic signals of various shapes such as sinusoids, triangular, rectangular and pulses are often needed to perform different types of operation. While the term oscillator is generally referred to as generator of sinusoid signals, a rectangular wave generator is more commonly known as a mulitivibrator. An oscillator generates an ac output signal without requiring any form of input signal. The signal - generating devices have certain fundamental elements in common, i.e. conversion of direct, or constant voltage to a particular periodically varying voltage, the manipulation of that voltage to a desired wave shape , and the precise control of the magnitude and frequency of that voltage. Generally an oscillator circuit consists of: • An amplifier circuit • A positive feedback circuit • Frequency determining components Three requirements for an oscillation to sustain are: • Positive feedback • An initial input trigger to start oscillations • Barkhausen criterion: loop gain ( 𝛽𝛽𝛽𝛽 ) = 1 Barkhausen Criteria Consider the feedback circuit shown in figure 4.1 below. A β + - V o = AV i V f = β ( Av i ) + - V i + - + - V f = β ( Av i ) Fig.4.1: Feedback used as an oscillator circuit From feedback analysis and the figure above, the loop gain is given by 𝛽𝛽𝛽𝛽 (product of the gain of the base amplifier and the feedback gain). If the circuits of the base amplifier and feedback network provide 𝛽𝛽𝛽𝛽 of correct magnitude and phase, 𝑉𝑉 𝑓𝑓 can be made equal to 𝑉𝑉 𝑖𝑖 . This results in to the feedback voltage to be the proper input voltage to drive the amplifier and feedback circuits. Hence, it sustains the loop operation i.e. the output waveform exists due to the feedback voltage if the following condition is met. Chapter Four Oscillators and Wave Shaping Circuits
Adama University EEng 2202 Electrical Engineering 2 𝛽𝛽𝛽𝛽 = 1 … … … … … … … … … … … … … … 4.1 This is known as the Barckhausen criterion for oscillation. 4.2 Sinusoidal Oscillators Sinusoidal oscillators use sine wave generator circuits to produce signals ranging from a low audio frequency to very high radio and microwave frequencies, which are constructed using resistor, capacitor and/or inductor. Many low frequency type sine wave generators use resistors and capacitors to form their frequency determining network and are referred to as RC Oscillators (shown in fig.4.2 below). They produce up to 1 MHz sine wave signals. V in V out R p C p R S C S Fig. 4.2 RC Oscillator The frequency of RC oscillator circuit shown in figure 4.2 is given by: 𝜔𝜔 𝑟𝑟 = 1 𝑅𝑅 𝑃𝑃 𝐶𝐶 𝑃𝑃 … … … … … … … … … 4.2 Another type of sine wave generator circuit, which can produce high frequency waves (up to 500 MHz) uses inductors and capacitors. These are referred to as LC tank circuits.
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