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Chapter 13: Oscillators the Oscillator an Oscillator Is a Circuit That Produces a Periodically Oscillating Waveform on Its Output with Dc Input

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Chapter 13: Oscillators the Oscillator an Oscillator Is a Circuit That Produces a Periodically Oscillating Waveform on Its Output with Dc Input Electronic Devices Chapter 13: Oscillators The Oscillator An oscillator is a circuit that produces a periodically oscillating waveform on its output with dc input. The output voltage can be either sinusoidal or nonsinusoidal, depending on the type of oscillator. Two major classifications for oscillators are feedback oscillators and relaxation oscillators. An oscillator converts electrical energy from the dc power supply to periodic waveforms. A basic oscillator is shown in Figure 1. Figure 1 Feedback Oscillators Feedback oscillator operation is based on the principle of positive feedback. A fraction of output signal is returned to input with no net phase shift resulting in a reinforcement of the output signal. After oscillations are started, the loop gain is maintained at 1.0 to maintain oscillations. A feedback oscillator consists of an amplifier for gain (either a discrete transistor or an op-amp) and a positive feedback circuit that produces phase shift and provides attenuation, as shown in Figure 2. Figure 2: Basic elements of a feedback oscillator. Positive Feedback Positive feedback is characterized by the condition wherein a portion of the output voltage of an amplifier is fed back to the input with no net phase shift, resulting in a reinforcement of the output signal. The inphase feedback voltage (Vf) is amplified to produce the output voltage, which in turn produces feedback voltage; a loop is created 101 Assist. Prof. Dr. Hamad Rahman Electronic Devices in which the signal sustains itself and a continuous sinusoidal output is produced. This phenomenon is called oscillation. In some types of amplifiers, the feedback circuit shifts the phase 180o and an inverting amplifier is required to provide another 180o phase shift so that there is no net phase shift as shown in Figure 3. Figure 3: Positive feedback produces oscillation. Conditions of Oscillation Two conditions, illustrated in Figure 4, are required for a sustained state of oscillation: o 1. The phase shift around the feedback loop must be 0 . 2. The voltage gain, Acl, around the closed feedback loop (loop gain) must be 1. Figure 4: General conditions to oscillation. Start-Up Conditions Feedback oscillators require a small disturbance such as that generated by thermal noise to start oscillations. This initial voltage starts the feedback process and oscillations. The feedback circuit permits only a voltage with a frequency equal to selected frequency to appear in phase on the amplifier’s input. The voltage gain conditions for both starting and sustaining oscillation are shown in Figure 5, when oscillation starts at t0, the condition Acl>1 causes the sinusoidal output voltage amplitude to build up to a desired level. Then Acl decreases to 1 and maintains the desired amplitude. 102 Assist. Prof. Dr. Hamad Rahman Electronic Devices Figure 5 The Wien-Bridge Oscillator One type of sinusoidal feedback oscillator is the Wien-bridge oscillator. RC feedback is used in various lower frequency (up to 1 MHz) sine-wave oscillators. The Wien-bridge is the most widely used type of RC feedback oscillator for this range of frequencies. Figure 6: A lead-lag circuit and its response curve. At resonant frequency (fr) the attenuation (Vout/Vin) of the circuit is 1/3. The lead-lag circuit is used in the feedback of Wien-Bridge oscillator as shown in Figure 6(a). It gives 0o phase shift and 1/3 attenuation at resonant frequency. The basic Wien-bridge uses the lead-lag network to select a specific frequency that is amplified. The voltage-divider sets the gain to make up for the attenuation of the feedback network. Figure 7 103 Assist. Prof. Dr. Hamad Rahman Electronic Devices The non-inverting amplifier must have a gain of exactly 3.0 as set by R1 and R2 to make up for the attenuation. If it is too little, oscillations will not occur; if it is too much the sine wave will be clipped. Wien-Bridge Oscillation Conditions The phase shift around the positive feedback loop must be 0° and the gain around the loop must equal unity (1). The 0° phase-shift condition is met when the frequency is fr because the phase shift through the lead-lag circuit is 0° and there is no inversion from the noninverting (+) input of the op-amp to the output. Figure 8 Wien-Bridge Oscillator Startup The loop gain should be greater than 1 at startup to build up output. Figure 9 Relaxation Oscillator A simple relaxation oscillator that uses a Schmitt trigger is the basic square-wave oscillator. The two trigger points, UTP and LTP are set by R2 and R3. The capacitor charges and discharges between these levels: R3 R3 VUTP = +Vmax ( ) VLTP = −Vmax ( ) R2 + R3 R2 + R3 104 Assist. Prof. Dr. Hamad Rahman Electronic Devices The period of the waveform is given by: 2R3 T = 2R1C ln (1 + ) R2 Figure 10: A square-wave relaxation oscillator. Noise Noise is a random fluctuation in an electrical signal. Noise in electronic devices varies greatly, as it can be produced by several different effects. Noise is a fundamental parameter to be considered in an electronic design as it typically limits the overall performance of the system. Noise is a purely random signal, the instantaneous value and/or phase of the waveform cannot be predicted at any time. Figure 11: Sine wave with superimposed noise. Noise can either be generated internally in the op-amp, from its associated passive components, or superimposed on the circuit by external sources. External refers to noise present in the signal being applied to the circuit or to noise introduced into the circuit by another means, such as conducted on a system ground or received on one of the many antennas formed by the traces and components in the system. Types of internal Noise . Thermal Noise . Shot Noise . Flicker Noise . Burst Noise . Avalanche Noise Some or all of these noises may be present in a design, presenting a noise spectrum unique to the system. It is not possible in most cases to separate the effects, but knowing general causes may help the designer optimize the design, minimizing noise in a particular bandwidth of interest. 105 Assist. Prof. Dr. Hamad Rahman Electronic Devices Thermal Noise Generated by the random thermal motion of charge carriers (usually electrons), inside an electrical conductor. It happens regardless of any applied voltage. Power spectral density is nearly equal throughout the frequency spectrum, approximately white noise. Figure 12 The root mean square voltage (VRMS) due to thermal noise, generated in a resistance (R) over bandwidth (BW), is given by: VRMS = √4푘BTRBW The noise from a resistor is proportional to its resistance and temperature. Lowering resistance values also reduces thermal noise. Shot Noise The name Shot Noise is short of Schottky noise, also called quantum noise. It is caused by random fluctuations in the motion of charge carriers in a conductor. Figure 13 Some characteristics of shot noise: . Shot noise is associated with current flow. It stops when the current flow stops. Shot noise is independent of temperature. Shot noise is spectrally flat or has a uniform power density, meaning that when plotted versus frequency it has a constant value. Shot noise is present in any conductor. Flicker noise Flicker noise is also called 1/f noise. Its origin is one of the oldest unsolved problems in physics. It is present in all active and many passive devices. It is related to imperfections in crystalline structure of semiconductors, as better processing can reduce it. 106 Assist. Prof. Dr. Hamad Rahman Electronic Devices Some characteristics of flicker noise: . It increases as the frequency decreases, hence the name 1/f. It is associated with a dc current in electronic devices. It has the same power content in each octave (or decade). Burst noise . Burst noise consists of sudden step-like transitions between two or more levels. It is related to imperfections in semiconductor material and heavy ion implants. As high as several hundred microvolts. Lasts for several milliseconds. Burst noise makes a popping sound at rates below 100 Hz when played through a speaker - it sounds like popcorn popping, hence also called popcorn noise. Low burst noise is achieved by using clean device processing, and therefore is beyond the control of the designer. Avalanche noise Avalanche noise is created when a PN junction is operated in the reverse breakdown mode. Under the influence of a strong reverse electric field within the junction’s depletion region, electrons have enough kinetic energy. They collide with the atoms of the crystal lattice, to form additional electron-hole pair. These collisions are purely random and produce random current pulses similar to shot noise, but much more intense. When electrons and holes in the depletion region of a reversed-biased junction acquire enough energy to cause the avalanche effect, a random series of large noise spikes will be generated. The magnitude of the noise is difficult to predict due to its dependence on the materials. Figure 14 107 Assist. Prof. Dr. Hamad Rahman .
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