65:. A circuit that is oscillating will not amplify linearly, so desired signals passing through the stage will be distorted. In digital circuits, parasitic oscillations may only occur on particular logic transitions and may result in erratic operation of subsequent stages; for example, a counter stage may see many spurious pulses and count erratically.
61:(EMI) to other devices. In audio systems, parasitic oscillations can sometimes be heard as annoying sounds in the speakers or earphones. The oscillations waste power and may cause undesirable heating. For example, an audio power amplifier that goes into parasitic oscillation may generate enough power to damage connected
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In practice, feedback may occur over a range of frequencies (for example the operating range of an amplifier); at various frequencies, the phase of the amplifier may be different. If there is one frequency where the feedback is positive and the amplitude condition is also fulfilled – the system
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Similarly, impedance in the power supply can couple input to output and cause oscillation. When a common power supply is used for several stages of amplification, the supply voltage may vary with the changing current in the output stage. The power supply voltage changes will appear in the input
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Parasitic oscillation in an amplifier stage occurs when part of the output energy is coupled into the input, with the correct phase and amplitude to provide positive feedback at some frequency. The coupling can occur directly between input and output wiring with stray
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Several measures are used to prevent parasitic oscillation. Amplifier circuits are laid out so that input and output wiring are not adjacent, preventing capacitive or inductive coupling. A metal shield may be placed over sensitive portions of the circuit.
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are used, high- and low-power stages are separated and ground return traces are arranged so that heavy currents don't flow in mutually shared portions of the ground trace. In some cases the problem may only be solved by introduction of another feedback
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In audio systems, if a microphone is placed close to a loudspeaker, parasitic oscillations may occur. This is caused by positive feedback, from amplifier's output to loudspeaker to sound waves, and back via the microphone to the amplifier input. See
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When parasitic oscillations occur, the designer can use the various tools of control loop engineering to correct the situation – to reduce the gain or to change the phase at problematic frequencies.
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of gain and phase vs. frequency. Using Bode plots, a design engineer checks whether there is a frequency where both conditions for oscillations are met: the phase is zero (
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between input and output. In some solid-state or vacuum electron devices there is sufficient internal capacitance to provide a feedback path. Since the
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Richard R Spencer & Ghausi MS (2003). Introduction to electronic circuit design. Upper Saddle River NJ: Prentice Hall/Pearson
Education. pp. 661.
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is common to both input and output, output current flowing through the impedance of the ground connection can also couple signals back to the input.
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control systems – the systems oscillated rather than performing their intended function, for example velocity control in engines. The
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P. Horowitz & W. Hill The Art of
Electronics Cambridge University Press (1980) Chapter 3, relating to operational amplifiers.
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Parasitic oscillation is undesirable for several reasons. The oscillations may be coupled into other circuits or radiate as
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within the passband of the amplifying device. A classic example is the
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These conditions can be expressed in mathematical terms using the
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was developed to address the problem of parasitic oscillation in
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of the inadvertent feedback path, must be equal to one, and the
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around the feedback loop, which is equal to the amplifier
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around the loop must be zero or a multiple of 360° (2π
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which plays well with a fresh battery, but squeals or "
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in an amplifying device. The problem occurs notably in
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157:. Another method used in control loop theory uses
126:gives the necessary condition for oscillation; the
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195:network, calculated and adjusted to eliminate the
267:A Desktop Reference of Hip Vintage Guitar Amps
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90:stage as positive feedback. An example is a
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165:) and the loop gain is 1 or greater.
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150:will oscillate at that frequency.
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420:Ordinary differential equations
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205:tuned radio frequency receivers
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270:. Hal Leonard. p. 220.
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59:electromagnetic interference
237:Whitaker, Jerry C. (2005).
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338:Araki, M., PID Control,
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240:The electronics handbook
294:DSP integrated circuits
405:Electronic oscillators
264:Weber, Gerald (1994).
188:printed circuit boards
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389:Categories
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201:Neutrodyne
173:Mitigation
159:Bode plots
110:Conditions
96:motorboats
80:inductance
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63:speakers
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