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In the diagram shown, the loop gain is the product of the gains of the amplifier and the feedback network, −Aβ. The minus sign is because the feedback signal is subtracted from the input. The gains A and β, and therefore the loop gain, generally vary with the frequency of the input signal, and so are usually expressed as functions of the ...
In practice, the loop gain is initially larger than unity. Random noise is present in all circuits, and some of that noise will be near the desired frequency. A loop gain greater than one allows the amplitude of frequency to increase exponentially each time around the loop. With a loop gain greater than one, the oscillator will start.
As long as the gain of the amplifier is high enough that the total gain around the loop is unity or higher, the circuit will generally oscillate. In RC oscillator circuits which use a single inverting amplifying device, such as a transistor, tube, or an op amp with the feedback applied to the inverting input, the amplifier provides 180° of the ...
Open-loop gain is finite in real operational amplifiers. Typical devices exhibit open-loop DC gain exceeding 100,000. So long as the loop gain (i.e., the product of open-loop and feedback gains) is very large, the closed-loop gain will be determined entirely by the amount of negative feedback (i.e., it will be independent of open-loop gain). In ...
A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is fixed relative to the phase of an input signal. Keeping the input and output phase in lockstep also implies keeping the input and output frequencies the same, thus a phase-locked loop can also track an input frequency.
Circuits with hysteresis are based on positive feedback. Any active circuit can be made to behave as a Schmitt trigger by applying positive feedback so that the loop gain is more than one. The positive feedback is introduced by adding a part of the output voltage to the input voltage.
Figure 10: Amplitude diagram of a 10th-order electronic filter plotted using a Bode plotter. The Bode plotter is an electronic instrument resembling an oscilloscope, which produces a Bode diagram, or a graph, of a circuit's voltage gain or phase shift plotted against frequency in a feedback control system or a filter. An example of this is ...
Tuning gain and noise present in the control signal affect the phase noise; high noise or high tuning gain imply more phase noise. Other important elements that determine the phase noise are sources of flicker noise (1/f noise) in the circuit, [8] the output power level, and the loaded Q factor of the resonator. [9] (see Leeson's equation).