LC feedback Oscillator


Posted on Feb 5, 2014

These are resonant circuit or tank circuit oscillators. They are commonly use to produced high frequencies as ranging from I MHz to 500 MHz Hence they are also known as RF oscillators. These oscillators are used in RF generators, radio transmitters and receivers. Some popular IC feedback oscillators are of the following types. Hartley oscillator i


LC feedback Oscillator
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s very popular and is commonly used as a local oscillator in radio receivers. The main advantages of this oscillator is wide range of frequencies and easy to use. Figure shows the circuit of Hartley oscillator. Its tank circuit consists of two coils L1 and L2. The coil L1 is inductively coupled to coil L2 and the combination works as an auto transformer. A coil called radio frequency choke RFC is connected between the collector and the Vcc supply. It acts as load for the collector and also permits an early flow of D. C current but blocks AC current. The feedback between the output and input circuit is accomplished, through auto transformer actions which also introduces a phase shift of 180o. The phase reversal between the output and input voltage occurs because they are taken from the opposite end of the coil L1 and L2 with respect to the tap, which is grounded. It may be observed that the tap on the combination L1 and L2 coils is actually connected to the transistor emitter terminal via ground and through the capacitor CE. As the transistor also introduces a phase shill of 180 °, therefore the total phase shift is 360 ° and hence the feedback is positive. As the capacitor Cc connected between the collector and the tuned circuit permits only the A. C currents to pass to the tank circuit. It means capacitor Cc blocks the D. C current. The capacitor CB is called blocking capacitor it further blocks the D. C current from...




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