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Presentation On Oscillator Circuits

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Published in: Electronics
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Oscillator Circuits

Nithya P / Ajman

5 years of teaching experience

Qualification: B Tech in Electronics And Communication Engineering, MBA in E Business

Teaches: Electronics, Physics, Engineering, Electrical Technology, Chemistry, Computer Science, Maths, Mathematics

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  1. Electronic Devices Ninth Edition Floyd LO-5 ELE-3413: Electronics-Il
  2. SurAf' Feedback Oscillators Oscillators are electronic circuits that produce a periodic waveform with only the dc supply voltage as an input. In a feedback oscillator, a fraction of the output is returned with no net phase shift. If the feedback circuit returns the signal out of phase, an inverting amplifier produces positive feedback. e ctranie Devices. 91/' edition mas L. Floyd Out of phæse Inverting circuit 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  3. SurAf' Feedback Oscillators Feedback oscillators require a small disturbance such as that generated by thermal noise to start oscillations. This initial voltage starts the feedback process and oscillations. Computer simulations, such as Multisim, use digital signals, which do not have thermal noise. This often creates a problem for computer simulations of oscillators. e ctranie Devices. 91/' edition mas L. Floyd 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  4. SurAf' The Wien-Bridge Oscillator RC feedback is used in various lower frequency sine-wave oscillators. The text covers three: the Wien-bridge oscillator, the phase-shift oscillator, and the twin-T oscillator. The feedback circuit in a Wien-bridge uses a lead-lag circuit. When the R's and C's have equal values, the output will be 1/3 of the input at only one frequency and the phase shift at this frequency will be 00. e ctranie Devices. 91/' edition mas L. Floyd 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  5. SurAf' The Wien-Bridge Oscillator The basic Wien-bridge uses the lead-lag network to select a specific frequency that is amplified. The voltage-divider sets the gain to make up for the attenuation of the feedback network. The noninverting amplifier must have a gain of exactly 3.0 as set by RI and R2 to make up for the attenuation. If it is too little, oscillations will not occur; if it is too much the sine wave will be clipped. e ctranie Devices. 91/' edition mas L. Floyd Voltage- divider Lead-lag network 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  6. The Wien-Bridge Oscillator B R2/(Rl + R2) 2R2 + R2 2R2 Positive feedback loop Phase shift Lead-lag (a) The phase shift arounkl the loop is 00. e ctranie Devices. 91/' edition mas L. Floyd 3R2 Loop gain — Lead-lag (b) The voltage gain amund the loop is 1. Lead-lag 3R2 + R3 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  7. SurAf' The Wien-Bridge Oscillator To produce the precise gain required, the Wien bridge needs some form of automatic gain control (AGC). One popular method is shown here and uses a JFET transistor. The key elements of the AGC circuit are highlighted in yellow. The diode charges C3 to the negative peak of the signal. This develops the gate bias voltage for the JFET that is related to the output level. e ctranie Devices. 91/' edition mas L. Floyd 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  8. SurAf' The Wien-Bridge Oscillator The JFET is operated in the ohmic region and can change its resistance rapidly if conditions change. Recall from Chapter 8 that a JFET acts as a variable resistor in the ohmic region. If the output increases, the bias tends to be larger, and the drain-source resistance increases (and vice-versa). In the Wien-bridge, the JFET drain- source resistance controls the gain of the op-amp and will compensate for any change to the output. e ctranie Devices. 91/' edition mas L. Floyd (mA) 7 6 4 2 O mic re Ion 0 2 4 5 vas (V) 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  9. SurAf' The Wien-Bridge Oscillator When the R's and C 's in the feedback circuit are equal, the 1 frequency of the bridge is given by f Example; What is fr for the Wien bridge? Solution: 1 21tRC 1 = 48.9 kHz - 271(680 nF) e ctranie Devices. 91/' edition mas L. Floyd 4.7 nF 680 0 680 Q 27tRC Q, 4.7 nF IOkQ 1.0kQ 10kO DI c, 1.0 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  10. EXAMPLE 16-1 FIGURE 16-12 Solution Determine the resonant frequency for the Wien-bridge oscillator in Figure 16—12. Also, calculate the setting for Rf assuming the internal drain-source resistance, ra, of the JFET is 500 0 when oscillations are stable. 10 kn 0.01 gF IN914 2NS458 001 "F 1.0kn For the lead-lag circuit, RI frequency is 2nRC IO„F —R— 10kO and Cl — G - c - 0.01 UF. The - 1.59kHz e ctranie Devices. 91/' edition mas L. Floyd The closed-loop gain must be 3.0 for oscillations to be sustained. For an inverting amplifier, the gain expression is the same as for a noninverting amplifier. Ri is composed of R3 (the source resistor) and Substituting, Rearranging and solving for RI, = (3 - + 5000) = 3.0kQ Upper Saddle River, NJ. 07458.
  11. SurAf' The Phase-Shift Oscillator The phase-shift oscillator uses three RC circuits in the feedback path that have a total phase shift of 1800 at one frequency — for this reason an inverting amplifier is required for this circuit. Even with identical R's and C 's, the phase shift in each RC circuit is slightly different because of loading effects. When all R's and C 's are equal, the feedback attenuates the signal by a factor of 1/29. e ctranie Devices. 91/' edition mas L. Floyd 1 29 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  12. SurAf' The Phase-Shift Oscillator Conditions for oscillation with the phase-shift oscillator is that if all R's and C 's are equal, the amplifier must have a gain of at least 29 to make up for the attenuation of the feedback circuit. This means that Rf/R3 29. Under these conditions, the frequency of oscillation is given by 1 2aC6RC e ctranie Devices. 91/' edition mas L. Floyd 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  13. (a) Determine the value of Rynecessary for the circuit in Figure 16—14 to operate as an oscillator. (b) Determine the frequency of oscillation. 0.001 "F 0.001 "F 10 kn c, 0001 "F 10 kn 10 kn Solution FIGURE 16-14 = 29, and B = R3 = 1/29 = R3/Rf Therefore, = 29 29R3 = 29(10kO) = 290k0 = R and Cl = C2 = Cy = C. Iherefore, 2rv6RC — BF) z G kHz NJ. 07458. ectronie Deviec mas L Floyd
  14. SurAf' The Phase-Shift Oscillator Multisim can simulate the phase-shift oscillator, but has difficulty starting. In the Multisim file for Example 16-3, a switch is provided to provide a voltage spike to start oscillations. This is not needed in the actual circuit. e ctranie Devices. 91/' edition mas L. Floyd tCEAL CFA',p 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  15. SurAf' The Phase-Shift Oscillator Design a phase-shift oscillator for a frequency of 800 Hz. The capacitors are to be 10 nF. Solution; Start by solving for the resistors needed in the feedback circuit: 1 = 8.12kQ 27tC6frc - 27tv6(800 nF) (Use 8.2 m.) Calculate the feedback resistor needed: RE 29R = 238 kQ. The following slide shows the Multisim check. e ctranie Devices. 91/' edition mas L. Floyd 238 kQ IOnF c: IOnF 8.2 kQ IOnF 8.2 8.2 kQ 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  16. SurAf' The Phase-Shift Oscillator Because the Rfis not precise, you will see the output "grow" in Multisim. In actual circuits, you can use a potentiometer to adjust a precise gain, but the circuit will be sensitive to temperature change. Key = Spaze I DEA L _CPAM p e ctranie Devices. 91/' edition mas L. Floyd V out 1 cnF 8.2kn 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  17. SurAf' The Phase-Shift Oscillator You can also use back-to back zener diodes to limit the output. The output is limited to about 7 v pp with IN4372A (3.0 V) zeners. Space e ctranie Devices. 91/' edition mas L. Floyd 1 onr IN4ä72A IN4372A 2012 Education. Upper saddle River, NJ. 07458. All rights reserved.
  18. SurAf' The Colpitts Oscillator LC feedback oscillators use resonant circuits in the feedback path. A popular LC oscillator is the Colpitts oscillator. It uses two series capacitors in the resonant circuit. The feedback voltage is developed across Cl. The effect is that the tank circuit is "tapped". Usually Cl is the larger capacitor because it develops the smaller voltage. e ctranie Devices. 91/' edition mas L. Floyd Out q In 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  19. SurAf' The Colpitts Oscillator The resonant frequency is found by fr = 27t LCT If Q > 10, this formula gives good results. Recall that the total capacitance of two series capacitors is the product-over-sum of the individual capacitors. Therefore, 27t L For Q < 10, a correction for Q is e ctranie Devices. 91/' edition mas L. Floyd 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  20. The Colpitts Oscillator + Vcc Amplifier Feedback circuit = 1. SinceB = C2/Cl, As you know, a condition for oscillation is AvB ectranie Devices. 91/' edirion mas L. Floyd e River, NJ. 07458.
  21. EXAMPLE 16-3 Solution e ctranie Devices. 91/' edition mas L. Floyd (a) Determine the frequency for the oscillator in Figure 16—21. Assume there is negli- gible loading on the feedback circuit and that its Q is greater than 10. (b) Find the frequency if the oscillator is loaded to a point where the Q drops to 8. FIGURE 16-21 .12 v c, c. 0.1 "F (0.1 LCF) 0.01 "F = 0.0091 0.11 = 7.46 kHz = = 7.40 kHz River, NJ. 07458. All rights reserved.
  22. SurAf' The Hartley Oscillator The Hartley oscillator is similar to the Colpitts oscillator, except the resonant circuit consists of two series inductors (or a single tapped inductor) and a parallel capacitor. The frequency for Q 10 is 1 1 27t lurc 211 (4+12)C One advantage of a Hartley oscillator is that it can be tuned by using a variable capacitor in the resonant circuit. e ctranie Devices. 91/' edition mas L. Floyd Out 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  23. The Hartley Oscillator Amplifier Feedback circuit In this circuit, the frequency of oscillation for Q > 10 is where LT = Ll + Ia. The inductors act in a role similar to Cl and C2 in the Colpitts to determine the attenuation, B, of the feedback circuit. ectronie De IQ mas L NJ. 07458.
  24. SurAf' The Crystal Oscillator Crystal oscillators are highly stable oscillators for demanding circuits such a radio transmitters. Crystals have very high Q. Manufacturers prepare natural crystals (usually quartz) by mounting a very thin slab between metal electrodes. When a small ac voltage is applied, the crystal oscillates at its own resonant frequency. The crystal acts as the resonant circuit for the modified Colpitts oscillator and stabilizes the oscillations. The capacitors still tap off a feedback signal to the CE amplifier. e ctranie Devices. 91/' edition mas L. Floyd + Vcc 1 XTAL 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  25. SurAf' Relaxation Oscillators Relaxation oscillators are characterized by an RC timing circuit and a device that periodically changes state. The triangular wave oscillator is an example. For this circuit, the device that changes states is a comparator with hysteresis (Schmitt trigger). The RC timing device is an integrator. The comparator output can be used as a square wave output. The trigger points set the triangle's peak-to-peak voltage: VLJTP = +1,' e ctranie Devices. 91/' edition mas L. Floyd A square wave can be taken as an output here. Comparator Integrator 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  26. Relaxation Oscillators FIGURE 16-28 A triangular-wave oscillator using two op-amps. out — UTP Comparator FIGURE 16-29 Waveforms for the circuit in Figure 16-28. VUTP Vmax COMP,out = — max [VUTP — (—Vmax)] e ctranie Devices. 91/' edition mas L. Floyd Integrator Comparator output VUTP VLT 0 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  27. SurAf' Relaxation Oscillators For the triangular wave generator, the frequency is found from: 4RlC R; ExampltWh at is the frequency of the circuit shown here? Solution; 4RlC R; - 4(82 nF) = 671 Hz e ctranie Devices. 91/' edition mas L. Floyd Comparator 82 IOkQ IOnF Integrator 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  28. SurAf' Relaxation Oscillators Normally, the triangle wave generator uses fast comparators to avoid slew rate problems. For non-critical applications, a 741 will work nicely for low frequencies (<2 kHz). The circuit here is one you can construct easily in lab. (The circuit is the same as Example 16-4 but with a larger C.) The waveforms are: Square wave 741 0.1 gF 10kQ 741 33 kQ 10kQ Triangle wave Both channels: 5 V/div 250 us/div e ctranie Devices. 91/' edition mas L. Floyd 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  29. SurAf' Relaxation Oscillators Another relaxation oscillator that uses a Schmitt trigger is the basic square-wave oscillator. The trigger points are set by R2 and R3. The capacitor charges and discharges between these levels: Vcrrp = +1,' LTP The period of the waveform is given by: T=2RlC1n e ctranie Devices. 91/' edition mas L. Floyd 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  30. The 555 Timer Addition of a few external parts forms a versatile astable multivibrator. The frequency and duty cycle are given by: 1.44 (RI Duty cycle = 100% RI +2R2 000 2 Vce — e ctranie Devices. 91/' edition mas L. Floyd SurAf' The 555 block diagram is shown. + Vec 0000 0000 Dt«hzg.ng 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  31. nteractivesMådéfiåå Wien Bridge Oscillator Wien Bridge Oscillator e ctranie Devices. 91/' edition mas L. Floyd 0 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  32. nteractivesMådéfiåå Phase shift oscillator RC Phase Shift Oscillator e ctranie Devices. 91/' edition mas L. Floyd 0 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  33. nteractivesMådéfiåå Twin T oscillator OSCILLATORS ANALOG ELECTRONICS e ctranie Devices. 91/' edition mas L. Floyd TWIN T OSCILLATOR Inymlnu\es 0 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  34. Feedback oscillator Relaxation oscillator Positive feedback e ctranie Devices. 91/' edition mas L. Floyd elected' An electronic circuit that operates with positive feedback and produces a time-varying output signal without an external input signal. An electronic circuit that uses an RC timing circuit to generate a nonsinusoidal waveform without an external input signal. The return of a portion of the output signal such that it reinforces and sustains the input signal. 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  35. 1. The Wien-bridge oscillator uses the network shown. If R's and C's are equal, the maximum v out will be phase- shifted by a. 00 b. 900 c. 1800 d. 2700 e ctranie Devices. 91/' edition mas L. Floyd c, 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  36. 2. The ohmic region of a JFET is the operating region for a JFET used in a Wien-bridge AGC circuit because the vc=ov a. resistance is constant b. resistance depends on VDS c. resistance depends on VG d. resistance depends on ID e ctranie Devices. 91/' edition mas L. Floyd -0.5 v % = -1.0 v vas (V) 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  37. 3. Assume the Wien-bridge oscillator shown is operating normally and the output voltage is a 6 v pp sine wave. VG should be a. I V sine wave pp b. 2 V sine wave pp d. -2.3 e ctranie Devices. 91/' edition mas L. Floyd 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  38. 4. The twin-t oscillator uses the network shown. At the oscillator frequency, the output of this network is phase- shifted by a. 00 b. 900 c. 1800 d. 2700 e ctranie Devices. 91/' edition mas L. Floyd 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  39. 5. The overall frequency response characteristic of the twin-t oscillator network shown is that of a a. low-pass filter b. high-pass filter c. band-pass filter d. notch filter e ctranie Devices. 91/' edition mas L. Floyd 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  40. 6. An example of an LC feedback oscillator is a a. phase-shift oscillator b. relaxation oscillator c. Colpitts oscillator d. Wien-bridge oscillator e ctranie Devices. 91/' edition mas L. Floyd 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  41. 7. In a crystal oscillator, the crystal acts like a a. band-pass filter b. resonant circuit c. notch filter d. power source e ctranie Devices. 91/' edition mas L. Floyd 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  42. 8. The waveforms at Voutl and V should be a out2 a. sawtooth wave and a sine wave b. triangle wave and a square wave c. sawtooth wave and a square wave d. triangle wave and a sine wave e ctranie Devices. 91/' edition mas L. Floyd 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  43. 9. The output waveform from this circuit should be a a. sawtooth wave b. triangle wave c. square wave d. sine wave e ctranie Devices. 91/' edition mas L. Floyd 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  44. 10. To make a basic astable multivibrator using a 555 timer, as a minimum you need a. one resistor and one capacitor b. one resistor and two capacitors c. two resistors and one capacitor d. two resistors and two capacitors e ctranie Devices. 91/' edition mas L. Floyd 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  45. Answers: 7. b ronic Devi«s. 9rh edition 10. c 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.
  46. Homework Solve the following problems from the textbook. Chapter 16, Pages 847-850, Problems: 7, 9, 10, 11, 12, 13, 14, 16, 17, 18, 19, 20, 21, 22 e ctranie Devices. 91/' edition mas L. Floyd 0 2012 Pearson Education. Upper Saddle River, NJ. 07458. All rights reserved.

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