Self excited oscillation refers to the constant and continuous oscillation generated by itself without external excitation signal.[1]If there is no input signal at the input end of the amplifier, there is still a certain amplitude and frequency output signal at the output end. This phenomenon is self-excited oscillation.
Self excited oscillation refers to the constant and continuous oscillation generated by itself without external excitation signal.[1]From a mathematical point of view, it is a free oscillation that occurs in some nonlinear systems.A typical example is the system described by Van der Pol equation. The equation form is mx ¨ - f (1-x2) x · - kx=0 (m>0, f>0, k>0), where x · and x ¨ are the first and second derivatives of variable x.The analysis shows that when the value of x is very small, the damping f is negative, so the motion is divergent;When the value of x is large, the damping f is positive, so the motion is attenuated.Therefore, no matter what initial conditions, the system motion tends to a continuous oscillation, namely self-excited oscillation.[2]
Conditions for generating self-excited oscillation
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Two conditions must be met at the same time to generate self-excited oscillation:
Where, A refers to the gain of the basic amplification circuit (open-loop gain), and F refers to the feedback coefficient of the feedback network
At the same time, the starting condition of | AF | slightly greater than 1 must be met
The basic amplification circuit must be composed of multi-stage amplification circuit to achieve high open-loop amplification. However, if negative feedback is added between stages of multi-stage amplification circuit, the phase shift of signal may make the negative feedback amplification circuit unstable and generate self-excited oscillation.The fundamental cause of self-excited oscillation in negative feedback amplifier circuit is the additional phase shift of AF (loop amplification factor)
The single-stage and two-stage amplification circuits are stable, while the three-stage or above negative feedback amplification circuits may generate self-excited oscillation as long as they have a certain feedback depth, because in the low frequency band and high frequency band, a frequency meeting the phase shift of 180 degrees (meeting the phase condition) can be found respectively. At this time, if the amplitude condition | AF |=1 is met, self-excited oscillation will be generated.Therefore, correction measures must be taken to destroy the self-excited oscillation for the negative feedback amplifier circuit with three or more stages, so as to achieve the purpose of stable operation of the circuit.
Compensation method
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The method of frequency compensation (also called phase compensation) can be used to eliminate self-excited oscillation.
Common compensation methods include capacitance lag compensation: select a small capacitance in parallel to the ground in the loop with the largest time constant in the amplification circuit, so that when the phase shift is 180 degrees, the amplitude of its high-frequency amplification factor drops below 0. Since this compensation is the phase lag corresponding to the frequency, it is called lag compensation.Others include RC lag compensation andMiller effectCompensation.
Free running oscillator
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Free running oscillatorMost of them are composed of amplifiers and positive feedback circuits.Oscillator is an energy conversion device, which can transform the DC form of energy into an alternating form through the oscillator. According to the form of self-excited oscillator generating alternating current, it can be divided into sinusoidal oscillator and non sinusoidal oscillator.
