22-01-2013, 02:30 PM
OSCILLATORS
OSCILLATORS[.ppt (Size: 608 KB / Downloads: 76)
OSCILLATORS:
Condition for oscillations. RC-phase oscillator with transistor and FET, Hartley and colpitts oscillators, Wien bridge oscillator, Crystal oscillators, Frequency and amplitude stability of oscillators.
The oscillators
Oscillator is a circuit that produce a continuous signal/waveform on its output with only the dc supply voltage as an input.
The output voltage can be either sinusoidal or non sinusoidal depending on the type of oscillator.
Instabilities, Oscillations and Oscillators
If positive feedback is applied to an amplifier, the feedback signal is in phase with the input, a regenerative situation exists.
If the magnitude of the feedback is large enough, an unstable circuit is obtained.
To achieve the oscillator circuit function, we must ensure an unstable situation. In addition we need to develop the oscillatory power at a desired frequency, with a given amplitude and with excellent constancy of envelope amplitude and frequency.
The design of good oscillators can be quite demanding because the governing equations of an oscillator are nonlinear, differential equations. Consequently oscillator analysis and design are not as advanced as that for linear circuits.
Typical oscillator analysis involves reasonably simple approximate analyses of linearized or piecewise-linear-circuit models of the oscillator together with perturbations and power series techniques.
There are a few oscillator circuits that can be solved exactly.
Amplitude Stability
In order to start the oscillation, the loop gain is usually slightly greater than unity.
LC oscillators in general do not require amplitude stabilization circuits because of the selectivity of the LC circuits.
In RC oscillators, some non-linear devices, e.g. NTC/PTC resistors, FET or zener diodes can be used to stabilized the amplitude
Wien-Bridge Oscillator
At resonant frequency, fr, phase shift through the circuit is 0o and the attenuation is 1/3
Below fr the lead circuit dominates and the output leads the input
Above fr, the lag circuit dominates and output lags the input
Positive feedback condition for Oscillation
To produce a sustained sinusoidal output (oscillate):
Phase shift around the positive feedback loop must be 0o
Gain around the loop must be at least unity (1)
0o phase-shift condition - met when the frequency is fr because the phase shift through the lead-lag circuit is 0o & no inversion from non inverting (+) input of the op-amp to the output