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aan, , Sinusoidal Oscillators m 367, , If there were no losses in the tank circuit, jt ereys the interchange of energy between L, , - sofia indefinitely. In a practical tank cir- “resist and radiation losses in the coil and, a in the capacitor. During each cycle, a small, ‘ne originally imparted energy is used up to over> losses. The result is that the amplitude of oscilent decreases gradually and eventually it becomes, en all the energy is consumed as losses. Therefore,, cuit by itself will produce damped oscillations as, inFig. 14.3., , | Frequency of oscillations. The frequency of oscilla- a, nthe tank circuit is determined by the constants of the ‘Fig. 143, cLand C. The actual frequency of oscillations is, , nant frequency (or natural frequency) of the tank circuit given by :, , pp earan dike n!, " 2n VLC, ls clear that fr, , Nicene eas €quency of oscillations in the“tank circuit is inversely proportional to L and G, ehrge fully - ee Ifa large value of capacitor is used, it will take longer for the capacitor, Rui op equival 286 longer to discharge. This will lengthen the period of oscillations in the tank, Neue ow 4 ently lower its frequency, With a large value of inductance, the opposition to change, fee 'S greater and hence the time required to complete each cycle will be longer. There, sy), Seater : . :, *slliong in aaa of inductance, the longer is the period or the lower is the frequency of, circuit,, , , , , , , , , , , , , , , , , , Scanned with CamScanner

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v, , s of Electronics tinuous und, auto luce con ‘am,, ecircuit will prod ped Matas, , 368 = Principle, , , , , , , , , , , , , , , , C led, th, If these conditions are fulfil, , Fig. 14.4. d, 14.5. Positive Fee, , - - Mn,, k Amplifier — Oscillatoy \, c Z a, — feedbach can act as an oscillator ; esi, sitive ig. 14.5 shows a transistor amplifies fan », ., , ii,, <a \\ *, Out, , per, , ; ifier with pro, A transistor amplifier with / | signal source., , Si i y extemal, oscillations without any ext, 180°, , , , , , 4—— |FEEDBACK, , , , , , "BN, , 180°, , Fig. 14.5, , feedback. Remember that a positive feedback amplifier is one that produces a feedback voltage (V; ) that is in phase with the original input, signal. As you can see, this condition is met in the circuit shown in Fig., 145. A phase shift of 180° is produced by the amplifier and a further, phase shift of 180° is introduced by feedback network. Consequently,, the signal is shifted by 360° and fed to the input i.e., feedback voltage is, in phase with the input signal., , (i) We note that the circuit shown in Fig. 14.5 is producing oscilJations in the output. However, this circuit has an input signal. This is, inconsistent with our definition of an oscillator i.e., an oscillator is a, circuit that produces oscillations without any external signal source., , ty lds, , , , , , AMPLIFIER, , , , , , , , , , Fig. 14.6, , (ii) When we open the Switch § of Fig. 14.5, , the input signal (V,,) is removed, However Vic, to the input signal. The amplifier ;, , is met, , Set the circuit shown in Fig. 14.6. Tag, , is in phase with the original ee a itn’, will be amplified and sent to the output. th Signal in the same way that it did! i, , . : » The fe F outpure db, , the input. Therefore, the amplifier Teceives sitet Network sends a portion of a seis P 4, , This process will continue so long as the amplifie; armed ee ani cur el, , Tis turned on, Therefore, the amplifer ”, , » We, ‘ V, (which, will respond to this, , Scanned with CamScanner

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p a, Sinusoidal Oscillators 369, at with no external signal source. The following points may be noted carefully :, 4g) OWE |, , 3 eansistO" amplifer with proper positive feedback will work as an oscillator, coe i ‘ ‘ iieas ., \ needs only a quick trigger signal to start the oscillations. Once the oscillations, , , , , , , , , , , , , , , , , , , , , , , , , ul, , e circuit, , i The started, NO external signal source is needed., ee to get continuous undamped output from the circuit, the following condition must, " t mel:, mA, = 1 ih, ' A, =-voltage gain of amplifer without feedback, where m, = feedback fraction, ais relation is called Barkhausen criterion. This: condition will be explained in the Art. 14.7., , 1.6 essentials of Transistor Oscillator, , 147 shows the block diagram of an oscillator. Its essential components are :, , i) Tank circuit, Tt consists of inductance coil (L) connected in parallel with capacitor (C). The, scillationg in the circuit depends upon the values of inductance of the coil and capaci, i, , equency of 0:, ce of the capacitor., , \ii) Transistor amplifier. The transistor amplifier receives d.c. power from the battery and, ges it into a.c. power for supplying to the tank circuit. The oscillations occurring in the tank, iruit are applied to the input of the transistor amplifier. Because of the amplifying properties of the, istor, we get increased output of these oscillations., , , , ITRANSISTOR|, | AMPLIFIER’, , , , , , , , , , , , , , | FEEDBACK. . “, CIRCUIT, , , , , , , , , , Fig. 14.7, , it amplified output of oscillations is due to the d.c, power supplied by the battery. The output, ane can be supplied to the tank circuit to meet the losses., it j : Feedback circuit. The feedback circuit supplies a part of collector energy to the tank cir, in * ses, “ortect phase to aid the oscillations i.e. it provides positive feedback. ), , 7 et = ., , ... Explanation of Barkhausen Criterion, , Dies criterion is that in order to produce continuous undamped oscillations at the output of an, * the positive feedback should be such that :, , =1, , Once a: m, A, ., , be = this condition is set in the positive, “ained at the output immediately afte, , undamped oscillations, , feedback amplifier, continuous, , 1: connecting the necessary power supp!, , Scanned with CamScanner