Philadelphia University Faculty of Engineering Communication and Electronics Engineering. Amplifier Circuits-IV

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Module: Electronics II Module Number: 6503 Philadelphia University Faculty o Engineering Communication and Electronics Engineering Ampliier Circuits-IV Oscillators and Linear

A repetitive input signal is not required except to synchronize oscillations in some applications.

Two major classiications or oscillators are eedback oscillators and relaxation oscillators.

1 Module: Electronics II Module Number: 6503 Philadelphia University Faculty o Engineering Communication and Electronics Engineering Ampliier Circuits-IV Oscillators and Linear Digital IC's: Oscillators: An oscillator is a circuit that produces a periodic waveorm on its output with only the dc supply voltage as an input. A repetitive input signal is not required except to synchronize oscillations in some applications. The output voltage can be either sinusoidal or nonsinusoidal, depending on the type o oscillator. Two major classiications or oscillators are eedback oscillators and relaxation oscillators. ) Feedback Oscillators: It returns a raction o the output signal to the input with no net phase shit, resulting in a reinorcement o the output signal. Ater oscillations are started, the loop gain is maintained at.0 to maintain oscillations. A eedback oscillator consists o an ampliier or gain (either a discrete transistor or an opamp) and a positive eedback circuit that produces phase shit and provides attenuation. Lecturer: Dr. Omar Daoud Part IV

2 Module: Electronics II Module Number: 6503 ) Relaxation Oscillators: It uses an RC timing circuit to generate a waveorm that is generally a square wave or other nonsinusoidal waveorm. Typically, a relaxation oscillator uses a Schmitt trigger or other device that changes states to alternately charge and discharge a capacitor through a resistor. Positive Feedback Oscillator: Positive eedback is characterized by the condition wherein an in-phase portion o the output voltage o an ampliier is ed back to the input with no net phase shit. The in-phase eedback voltage, V is ampliied to produce the output voltage, which in turn produces the eedback voltage. That is, a loop is created in which the signal sustains itsel and a continuous sinusoidal output is produced. This phenomenon is called oscillation. Two conditions are required or a sustained state o oscillation:. The phase shit around the eedback loop must be eectively 0.. The voltage gain, A cl, around the closed eedback loop (loop gain) must equal (unity). Lecturer: Dr. Omar Daoud Part IV

Module: Electronics II Module Number: 6503 The Phase-Shit Oscillator: Each o the three RC circuits in the eedback loop can provide a maximum phase shit approaching 90.

3 Module: Electronics II Module Number: 6503 The Phase-Shit Oscillator: Each o the three RC circuits in the eedback loop can provide a maximum phase shit approaching 90. Oscillation occurs at the requency where the total phase shit through the three RC circuits is 80. The inversion o the op-amp itsel provides the additional 80 to meet the requirement or oscillation o a 360 (or 0 ) phase shit around the eedback loop. The attenuation, B, o the three-section RC eedback circuit is B 9 where B = R 3 /R. The requency o oscillation ( r ) in the ollowing equation, where R = R = R 3 = R and C = C = C 3 = C. r 6RC Ex. (a) Determine the value o R necessary or the given circuit to operate as an oscillator. (b) Determine the requency o oscillation. Solution: (a) (b) R3 Acl 9, and B R 90k 9 R r 6. 5kHz 6 0k0.00F Lecturer: Dr. Omar Daoud Part IV 3

Module: Electronics II Module Number: 6503 The Colpitts Oscillator: This type o oscillator uses an LC circuit in the eedback loop to provide the necessary phase shit and to act as a resonant ilter

4 Module: Electronics II Module Number: 6503 The Colpitts Oscillator: This type o oscillator uses an LC circuit in the eedback loop to provide the necessary phase shit and to act as a resonant ilter that passes only the desired requency o oscillation. The approximate requency o oscillation is the resonant requency o the LC circuit and is established by the values o C l,c, and L according to this amiliar ormula: Q r LC Q T Q0 LCT CC CT C C C The attenuation, B= C C A v = C Ex. (a) Determine the requency or the oscillator in the given Figure. Assume there is negligible loading on the eedback circuit and that its Q is greater than 0. (b) Find the requency i the oscillator is loaded to a point where the Q drops to 8. Solution: (a) 0.F 0.0F CT 0.009F 0.F 0.0F r LC (b) T 50mH 0.009F 8 r 7.46kHz 7. 4kHz kHz Lecturer: Dr. Omar Daoud Part IV 4

5 Module: Electronics II Module Number: 6503 The Wien-Bridge Oscillator: A part o the Wien-bridge oscillator is a lead-lag circuit like that shown in the given Figure below. R and C together orm the lag portion o the circuit; R and C orm the lead portion. The operation o this lead-lag circuit is as ollows: At lower requencies, the lead circuit dominates due to the high reactance o C. As the requency increases, X C decreases, thus allowing the output voltage to increase. At some speciied requency, the response o the lag circuit takes over, and the decreasing value o X C causes the output voltage to decrease. At resonant ( r ), the attenuation (V out /V in ) o the circuit is /3 i RC R = R and X C = X C as stated by the ollowing equation: V out /V in =/3 Crystal-Controlled Oscillator: It contains a quartz crystal that is used to control the requency o operation. The crystal vibrates at a constant rate when it is exposed to an electric ield, this vibration depends on the size o the crystal itsel. Voltage-Controlled Oscillator: It is a relaxation oscillator whose requency can be changed by a variable dc control voltage. Lecturer: Dr. Omar Daoud Part IV 5

6 Module: Electronics II Module Number: 6503 Linear-Digital ICs Among the linear/digital ICs are comparator circuits, digital/analog converters, interace circuits, timer circuits, voltage-controlled oscillator (VCO) circuits, and phaselocked loops (PLLs). The comparator circuit is one to which a linear input voltage is compared to another reerence voltage, the output being a digital condition representing whether the input voltage exceeded the reerence voltage. Circuits that convert digital signals into an analog or linear voltage, and those that convert a linear voltage into a digital value, are popular in aerospace equipment, automotive equipment, and compact disk (CD) players, among many others. Interace circuits are used to enable connecting signals o dierent digital voltage levels, rom dierent types o output devices, or rom dierent impedances so that both the driver stage and the receiver stage operate properly. Timer ICs provide linear and digital circuits to use in various timing operations, as in a car alarm, a home timer to turn lights on or o, and a circuit in electromechanical equipment to provide proper timing to match the intended unit operation. The 555 timer has long been a popular IC unit. A voltage-controlled oscillator provides an output clock signal whose requency can be varied or adjusted by an input voltage. One popular application o a VCO is in a phase-locked loop unit, as used in various communication transmitters and receivers. Lecturer: Dr. Omar Daoud Part IV 6

Chapter.8: Oscillators

Chapter.8: Oscillators Objectives: To understand The basic operation of an Oscillator the working of low frequency oscillators RC phase shift oscillator Wien bridge Oscillator the working of tuned oscillator