DEFINITION: An oscillator is just an electronic circuit which converts dc energy into AC energy of required frequency. (Or) An oscillator is an electronic circuit which produces an ac output without any input. Classification of oscillators 1. Based on the frequency generated : Oscillator type AF Oscillators RF Oscillators VHF Oscillators UHF Oscillators Microwave Oscillators Frequency range Few Hz - 20 khz. 20 khz 30 MHz 30 MHz 300 MHz 300 MHz 3 GHz. Above 3 GHz. 2. Based on the Auxiliary Oscillatory circuit used : (a) RC oscillators (i) RC Phase-shift and (ii) Wein-bridge oscillators (b) LC Oscillators (i) Tuned collector (ii) Hartley oscillators (iii)colpitts oscillators (iv)crystal Oscillator. 3. Based on the generated waveform (a) Sine wave oscillators. (b) Non - sinusoidal oscillators. KONA LAKSHMANARAO 1
CONDITIONS REQUIRED FOR SUSTAINED OSCILLATIONS: The conditions for sustained oscillations are 1. A transistor amplifier with positive feedback. 2. The total phase shift around the loop is 360 0 or 0 0. 3. Loop gain Aβ = 1 i.e.., A = voltage gain of amplifier without feedback. Β = Feedback fraction. NEED FOR AUDIO FREQUENCY OSCILLATOR: The oscillator operate in the audio frequencies is known as audio oscillator. In radio receivers, T.V receiver P.A. system and other appliances related to sound output consists of audio circuits. To service the audio circuits there is a need to have known audio signal. The known audio signals are generated using audio oscillators. Therefore the audio frequency oscillators are used in laboratories. Basically RC circuits are used in audio oscillators. RC PHASE SHIFT OSCILLATOR: As shows the circuit of a phase shift oscillator. It consists of a conventional single transistor amplifier and a RC phase shift network. The phase shift network consists of three sections R 1 C 1, R 2 C 2 and R 3 C 3. At some particular frequency fo, the phase shift of each section is 60º, so that the total phase-shift produced by the RC network is (3*60º)=180º. The frequency of oscillations is given by 1 f 2 RC 6 Where R1=R2=R3=R C1=C2=C3=C. Circuit Operation: When the circuit is switched on it produces oscillations. The output Eo of the amplifier is feedback to RC feedback network. KONA LAKSHMANARAO 2
This network produces phase shift of 180º and a voltage E1 appears at its output which is applied to the transistor amplifier. The feedback factor β=e 1 /E 0. The feedback factor β=1/29, has an important significance. For self-starting the oscillations, we must have Aβ>1, it means the gain A of the amplifier must be greater than 29 only then the oscillations can start. A phase shift of 180º is produced by the transistor amplifier. A further phase shift of 180º is produced by the RC network. As a result the phase shift around the loop is 360º. Phase Shift Network: A phase shift circuit essentially consists of an RC network. Fig.2.(b) shows a single section of RC network. It can be shown that alternating voltage V 1 across R leads the applied voltage V 1 by an angle θ. The value of θ depends upon the value of R and C. If resistance R is varied, the value of θ also changes KONA LAKSHMANARAO 3
If R were reduced to zero V 1 will lead V 1 by 90 that is θ=90º By adjusting the value of R the value θ can be obtained to 60º. Fig.2. (c) shows the three section of RC network. Each section produces a phase shift of 60º. Consequently, a total phase shift of 180º is produced i.e voltage V 2 leads the voltage V 1 by 180º. ADVANTAGES It does not require transformers or inductors. It can be used to produce very low frequencies. It provides good frequency stability. Pure sine wave output is possible.. DISADVANTAGES It is difficult for the circuit to start oscillations as the feedback is generally small. The circuit gives small output. It requires high voltage battery VCC 12V. NEED FOR RADIO FREQUENCY OSCILLATOR: The oscillators operates at frequencies typically from 200 KHz to few GHz is known as Radio frequency oscillators. Radio receiver, TV receiver and other communication receivers first stages are operates in the radio frequency range. To service these stages of receivers the RF oscillators is used. Basically LC circuits are used in radio frequency oscillators. HARTLEY OSCILLATOR: As shows the circuit of Hartley oscillator. The tank circuit is C, L 1 and L 2. The coil L 1 is inductively coupled to L 2, the combination functions as an autotransformer. The self-bias is provided here for biasing. The capacitor C b blocks the DC components. KONA LAKSHMANARAO 4
Circuit operation: When the power is ON, collector current starts rising and charges the capacitor C. When this capacitor is fully charged, it discharges through coils L 1 and L 2 setting up Oscillations. The oscillations across L 1 are applied to the base emitter junction and appears in the amplified form in the collector circuit. The coil L 2 couples the collector circuit energy back into the tank circuit by means of mutual inductance between L 1 and L 2. So energy is being continuously supplied to the tank circuit to overcome the losses. The phase shift produced in the tank circuit constituting of C, L 1 and L 2 is (i.e., phase shift between the voltages Developed across both the Inductors L 1 & L 2 is) 180 0. A further phase shift of 180º is produced by transistor circuit. In this way energy feedback to the tank circuit is in phase with oscillations. The frequency of oscillations is given by f 2 ( L 1 1 L ) C COLPITT S OSCILLATOR: Fig.1 shows the circuit of Colpitts oscillator. The tank circuit is made up of C 1, C 2 and L. 2 Circuit operation: When power is ON, collector current starts rising and charges the capacitor C 1 and C 2. When these capacitors are fully charged, it discharges through coil L 1 setting up oscillations. The oscillations across C 2 are applied to the base-emitter junction and appears in the amplified form in the collector circuit to supply losses to the tank circuit. KONA LAKSHMANARAO 5
The amount of feedback depends upon the values of C 1 and C 2. The phase of feedback is correct, the tank circuit comprising of L, C 1 and C 2 produce 180º phase shift. A further 180º phase shift is provided by the transistor. In this way feedback is properly provided to produce continuous undamped oscillations. The frequency of oscillations is given by f 2 1 LC T C1C 2 CT 1 2 Where, C C NEED OF SQUARE WAVE OSCILLATOR The oscillator generates square wave output is known as square wave oscillator. The digital circuit s works in synchronism with square wave signal known as clock signal. TO generate clock signals wave oscillators are needed. In such application square wave oscillators are used. ASTABLE MULTIVIBRATOR: In which the circuit is not stable in either state. It continuously oscillates from one state to the other. Circuit operation: The two transistors Q1 and Q2 should be of the same specifications. Rc1 and Rc2 are collector load resistances of Q1 and Q2. The values of Rc1 and Rc2 are same. C1 and C2 are the capacitors used for cross coupling. R1, C1 and R2, C2 decides the frequency of the circuit. KONA LAKSHMANARAO 6
Working: At t = 0-. Circuit is in OFF condition. At t = 0. Circuit is switched ON and both the transistors are in quasi stable state. At t = 0+. Because of small variations in the values of Q1 and Q2. One of the two transistors will go to ON state and other will go to OFF state. R1, C1 and R2, C2 decides the ON and OFF time periods of Q1 and Q2, and frequency of the circuit. From t = 0+ to t = T1 The circuit keeps one transistor switched ON and the other switched OFF. Suppose that initially, i.e., at t = 0+, Q2 is switched ON and Q1 is switched OFF. From t = 0+ to t = T1 State 1: Since Q2 is ON, the capacitor C2 charges through Rc1, and the charging voltage across C2 is Vcc. The capacitor C1 discharges through resistor R1. The voltage across C1, when it is about to start discharging is Vcc. As C1 discharges, the voltage at base of Q1 becomes more positive and cross the cutin voltage of Q1. At time t = T1, the positive increase of base voltage Exceeds the cut in voltage of Q1. Transistor Q1 starts conducting, and transistor Q2 becomes OFF. This now takes us to State 2 : Where Q1 is switched ON and Q2 is switched OFF. Since Q1 is ON, the capacitor C1 charges through Rc2, and the charging voltage across C1 becomes Vcc. The capacitor C2 discharges through resistor R2. The voltage across C2 when it is about to start discharging is Vcc. The time Period from t = T1 (+) to T2 (-) As C2 discharges, the voltage at base of Q2 becomes more positive and cross the cutin voltage. At time t = T2, the positive increase of base voltage exceeds the cut in voltage of Q2. Transistor Q2 starts conducting, and transistor Q1 becomes OFF. This now takes back to State 1, the mirror image of the initial state, where Q1 is switched off and Q2 is switched on. This now takes us to State 2 : Where Q1 is switched ON and Q2 is switched OFF. KONA LAKSHMANARAO 7
Since Q1 is ON, the capacitor C1 charges through Rc2, and the charging voltage across C1 becomes Vcc. The capacitor C2 discharges through resistor R2. The voltage across C2 when it is about to start discharging is Vcc. As C2 discharges, the voltage at base of Q2 becomes more positive and cross the cutin voltage. At time t = T2, the positive increase of base voltage exceeds the cut in voltage of Q2. Transistor Q2 starts conducting, and transistor Q1 becomes OFF. This now takes back to State 1, the mirror image of the initial state, where Q1 is switched off and Q2 is switched on. The cycle repeats. WAVE FORMS: KONA LAKSHMANARAO 8
UJT RELAXATION OSCILLATOR: An oscillator which produces non sinusoidal waveforms like square wave, rectangular wave, saw tooth wave, triangular wave etc.., is called as relaxation oscillator. The relaxation oscillator using UJT as shown in fig. The below fig shows the discharging of a capacitor through UJT can develop saw tooth output as shown in fig. When battery V bb is turned ON, the capacitor C charges through resistor R 1. During the charging period, the voltage across the capacitor rises in an exponential manner until it reaches the peak point voltage. At this instant of time, the UJT switches to its low resistance conducting mode and the capacitor is discharge between E and B 1. As the capacitor voltage flys back to zero, the emitter ceases to conduct and the UJT is switched OFF. The next cycle then begins, allowing the capacitor C to charge again. The frequency of the output saw tooth wave can be varied by changing the value of R 1 since this controls the time constant R 1 C of the capacitor charging current. APPLICATIONS OF RC PHASE SHIFT OSCILLATORS: 1. Signal Generators. 2. Function generators. 3. AFO APPLICATIONS OF HARTELY AND COLPITTS OSCILLATORS: 1. Radio receivers as a local oscillators. 2. T.V receivers as a local oscillators. KONA LAKSHMANARAO 9
3. For induction and dielectric heating purpose. 4. High frequency applications. APPLICATIONS OF SQUARE WAVE OSCILLATORS: 1. Voltage to frequency converter. 2. Clock generator for logic signals. 3. In digital meters. 4. In SMPS. APPLICATIONS OF RELAXATION OSCILLATORS: 1. For generation of saw tooth wave form in CRO. 2. In timer circuits to drive SCR, DIAC, TRIAC. KONA LAKSHMANARAO 10