Unit-8 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.

Frequency Stability The frequency stability of an oscillator is defined as Use high stability capacitors, e.g. silver mica, polystyrene, or Teflon capacitors and low temperature coefficient inductors for high stable oscillators.

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

Conditions for Phase shift around the feedback loop must be 0 o Voltage gain, A cl, around the closed feedback loop (loop gain) must equal 1 (unity) – The voltage gain around the closed feedback loop (A cl ) is the product of amplifier gain (A v ) and the attenuation (B) of the feedback circuit A cl = A v B Start-Up Conditions For oscillation to begin, A cl around the positive feedback loop must be greater than 1 so that the output voltage can build up to a desired level. Then A cl decrease to 1 and maintains the desired magnitude Oscillation

Three types of RC oscillators that produce sinusoidal outputs will be discussed : 1.Wienbridge oscillator 2.phase-shift oscillator 3.twin-T oscillator. Generally RC oscillators are used for frequencies up to about 1 MHz Wienbridge oscillator is most widely used for this range of frequencies

The Colpitts Oscillator

Uses an LC circuit in the feedback loop : To provide necessary phase shift To act as a resonant filter that passes only the desired frequency Approximate frequency of oscillation :

The Colpitts Oscillator The input impedance of transistor amplifier acts as a load on the resonant feedback circuit and reduce the quality factor, Q of the circuit When Q > 10, frequency = If Q < 10, f r is reduced significantly FET can be used in place of BJT to minimize the loading effect of the transistors input impedance. When connected to external load, f r may decrease because of a the reduction in Q.

The Hartley Oscillator The Hartley oscillator is similar to the Clapp and Colpitts. The tank circuit has two inductors and one capacitor. The calculation of the resonant frequency is the same. For Q > 10 : Attenuation, B : Loading of the tank circuit same as in Colpitts: Q is decreased and thus f r decrease

Phase Shift Oscillator Example i. Determine the value of R f necessary for the circuit above to operate as an oscillator ii. Determine the frequency of oscillation Given: C 1 =C 2 =C 3 = uF R 1 =R 2 =R 3 = 10 kΩ

Phase Shift Oscillator Solution i. A cl = 29, B = 1/29 = R 3 /R f, therefore : ii. C1=C2=C3 and R1=R2=R3. Ttherefore :

Phase Shift Oscillator The phase shift oscillator utilizes three RC circuits to provide 180º phase shift that when coupled with the 180º of the op- amp itself provides the necessary feedback to sustain oscillations. The gain must be at least 29 to maintain the oscillations. The frequency of resonance for the this type is similar to any RC circuit oscillator.

Wien-Bridge Oscillator Fundamental part : Lead-Lag circuit Lag circuit : R 1 & C 1 Lead circuit : R 2 & C 2 At resonant frequency, f r, phase shift through the circuit is 0 o and the attenuation is 1/3 Below f r the lead circuit dominates and the output leads the input Above f r, the lag circuit dominates and output lags the input

Wien-Bridge Oscillator (cont..) Positive feedback condition for Oscillation To produce a sustained sinusoidal output (oscillate): 1.Phase shift around the positive feedback loop must be 0 o 2.Gain around the loop must be at least unity (1) 0 o phase-shift condition - met when the frequency is f r because the phase shift through the lead-lag circuit is 0 o & no inversion from non inverting (+) input of the op-amp to the output

Wien-Bridge Oscillator (cont..) Resonant Frequency : At Resonant Frequency : R 1 = R 2 and X C1 = X C2 :

Crystal Oscillators If a piezoelectric crystal, usually quartz, has electrodes plated on opposite faces and a potential is applied between these electrodes, forces will be exerted on the bound charges within the crystal. If this device is properly mounted, deformations takes place within the crystal, and an electromechanical system is formed which will vibrate when properly excited. Frequencies ranging from a few kHz to a few hundred MHz. Q values range from several thousand to several hundred thousand. The equivalent circuit of a crystal is shown below L,C and R are analogs of mass, compliance (reciprocal of spring constant) and viscous-damping factor of the mechanical system. If we neglect R the impedance of the crystal is

where