Oscillators with LC Feedback Circuits LC Feedback elements

Oscillators with LC Feedback Circuits

• LC Feedback elements are used for the generation of higher frequencies of oscillation. • Because of lower unity gain frequency of most op amps, discrete transistors are often used as the gain elements in LC Feedback oscillators. • Colpitts, Clapp, Hartley, Armstrong and Crystal controlled oscillators are the examples

The Colpitts Oscillator • LC feedback circuit provides the required phase shift and acts as resonant filter that passes only the desired frequency of oscillation.

For the oscillator to be self starting, OR Loading Effect on Feedback Circuit Input impedance of the amplifier acts as a load on the resonant feedback circuit and reduces the quality factor of the circuit When Q>10, When Q<10, the resonant frequency is significantly reduced

The Hartley Oscillator

OR Loading Effect is the same as in the case of Colpitts, i. e, to reduce the quality factor hence reducing the resonant frequency.

The Armstrong Oscillator

• Uses transformer coupling for feedback provision of the signal voltage. • Sometimes called tickler oscillator in reference to transformer secondary of tickler coil. • Less common as compared to previous ones mainly because of transformer size and cost. • The frequency of oscillation is set by the inductance of primary winding, Lpri in parallel with C 1

Crystal Controlled Oscillators • Most stable and accurate type of feedback oscillators • Exhibits a very high Q (values of several thousand are typical) • Piezoelectric crystal is used in the feedback loop for the frequency control • Quartz crystal is normally used in electronic applications in the form of a quartz wafer • Piezoelectric Effect: when a changing mechanical stress is applied across the crystal to cause it to vibrate, a voltage is developed at the frequency of mechanical vibration • Conversely when an ac voltage is applied across the crystal, it vibrates at the frequency of applied voltage • The greatest vibration occurs at the natural resonant frequency.

Crystal used as series resonant circuit Maximum feedback is provided at series resonant frequency where impedance of the crystal is minimum. CC is used to fine tune the oscillator frequency by pulling the resonant frequency of the crystal slightly up or down