circuit that produces a periodic 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. • Two major classifications for oscillators are feedback oscillators and relaxation oscillators. o an oscillator converts electrical energy from the dc power supply to periodic waveforms. J-601-1448 , Lec#12 , Jan 2015
characterized by the condition wherein a portion of the output voltage of an amplifier is fed back to the input with no net phase shift, resulting in a reinforcement of the output signal. Basic elements of a feedback oscillator. J-601-1448 , Lec#12 , Jan 2015
shift around the feedback loop must be effectively 0°. 2. The voltage gain, Acl around the closed feedback loop (loop gain) must equal 1 (unity). • Two conditions: J-601-1448 , Lec#12 , Jan 2015
begin, the voltage gain around the positive feedback loop must be greater than 1 so that the amplitude of the output can build up to a desired level. • The gain must then decrease to 1 so that the output stays at the desired level and oscillation is sustained. • Initially, a small positive feedback voltage develops from thermally produced broad-band noise in the resistors or other components or from power supply turn- on transients. J-601-1448 , Lec#12 , Jan 2015
used for frequencies up to about 1 MHz. • The Wien-bridge is by far the most widely used type of RC feedback oscillator for this range of frequencies. The Wien-Bridge Oscillator Lead-lag circuit and its response curve • Basic Circuit J-601-1448 , Lec#12 , Jan 2015
of automatic gain control (AGC) 1- When dc power is first applied, both zener diodes appear as opens. 2- When the zeners conduct, they short out R3 and Acl = 3 • In some older designs, a tungsten lamp was used in the feed-back circuit to achieve stability. • A better method to control the gain uses a JFET as a voltage-controlled resistor in a negative feedback path. • As the voltage increases, the drain-source resistance increases. • The zener feedback is simple, it suffers from the nonlinearity of the zener diodes that occurs in order to control gain. J-601-1448 , Lec#12 , Jan 2015
the three RC circuits in the feedback loop can provide a maximum phase shift approaching 90°. • Oscillation occurs at the frequency where the total phase shift through the three RC circuits is 180°. • The inversion of the op-amp itself provides the additional 180° to meet the requirement for oscillation of a 360° (or 0°) phase shift around the feedback loop. J-601-1448 , Lec#12 , Jan 2015
twin-T filters has a low-pass response, and the other has a high-pass response. • The combined parallel filters produce a band-stop or notch response with a center frequency equal to the desired frequency of oscillation. J-601-1448 , Lec#12 , Jan 2015
are normally used in oscillators that require higher frequencies of oscillation. • Also, because of the frequency limitation (lower unity-gain frequency) of most op- amps, discrete transistors (BJT or FET) are often used as the gain element in LC oscillators. • Colpitts oscillator uses an LC circuit in the feedback loop to provide the necessary phase shift and to act as a resonant filter that passes only the desired frequency of oscillation. J-601-1448 , Lec#12 , Jan 2015
Loading of the Feedback Circuit Affects the Frequency of Oscillation Zin of the amplifier loads the feed-back circuit and lowers its Q, thus lowering the resonant frequency. A FET can be used in place of a BJT, as shown in Figure 16–19, to minimize the loading effect of the transistor’s input impedance. J-601-1448 , Lec#12 , Jan 2015
C2 are both connected to ground at one end, the junction capacitance of the transistor and other stray capacitances appear in parallel with C1 and C2 to ground, altering their effective values. • C3 is not affected, however, and thus provides a more accurate and stable frequency of oscillation. • The Clapp oscillator is a variation of the Colpitts with addition of C3 . J-601-1448 , Lec#12 , Jan 2015
is similar to the Colpitts except that the feedback circuit consists of two series inductors and a parallel capacitor • Loading of the tank circuit has the same effect in the Hartley as in the Colpitts; that is, the Q is decreased and thus fr decreases. J-601-1448 , Lec#12 , Jan 2015
LC feedback oscillator uses transformer coupling to feed back a portion of the signal voltage. • It is sometimes called a “tickler” oscillator in reference to the transformer secondary or “tickler coil” that provides the feedback to keep the oscillation going. • The Armstrong is less common than the Colpitts, Clapp, and Hartley, mainly because of the disadvantage of transformer size and cost. J-601-1448 , Lec#12 , Jan 2015
and accurate type of feedback oscillator uses a piezoelectric crystal in the feedback loop to control the frequency. • Quartz is one type of crystalline substance found in nature that exhibits a property called the piezoelectric effect. • When a changing mechanical stress is applied across the crystal to cause it to vibrate, a voltage develops at the frequency of mechanical vibration. • Conversely, when an ac voltage is applied across the crystal, it vibrates at the frequency of the applied voltage. • The greatest vibration occurs at the crystal’s natural resonant frequency, which is determined by the physical dimensions and by the way the crystal is cut. J-601-1448 , Lec#12 , Jan 2015
advantage of the crystal is that it exhibits a very high Q. • The impedance of the crystal is minimum at the series resonant frequency, thus providing maximum feedback. • a crystal is used as a series resonant tank circuit. • The crystal tuning capacitor, Cc is used to “fine tune” the oscillator frequency by “pulling” the resonant frequency of the crystal slightly up or down. Modes: • Piezoelectric crystals can oscillate in either of two modes—fundamental or overtone. • The fundamental frequency of a crystal is the lowest frequency at which it is naturally resonant. • The fundamental frequency depends on the crystal’s mechanical dimensions, type of cut, .. etc. • Usually it’s less than 20 MHz. • Overtones are approximate integer multiples of the fundamental frequency. • Many crystal oscillators are available in integrated circuit packages. J-601-1448 , Lec#12 , Jan 2015
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