Oscillators

Oscillators introduction and its types, phase shift oscillators and wein bridge oscillators,difference between phase shift and wein bridge, frequency stability, oscillators principle and conditions, block diagram of oscillators, block diagram of phase shift oscillators

1. Oscillators By Neha mannewar

2. Index Introduction Principles Block Diagram Condition Frequency Stability Types of Oscillators Phase Shift Oscillators Wein Bridge Oscillators Comparison between Phase shift and W ein bridge

3. Introduction Convert DC energy to AC energy at a very high frequency. If the feedback signal is large enough and has correct phase ,there will be an output signal even though there is no external input signal. The criteria is that the signal feedback to the input of the amplifier must be in phase . In-phase feedback is also called positive feedback , or regenerative feedback. It is unstable amplifier. Electronic Oscillators are divided into: Sinusoidal (or harmonic) oscillators –which produce an input using sine wave form. Non-sinusoidal (or relaxation) oscillators- the output is square , rectangular or saw tooth or pulse shape. Oscillators are widely applied in many digital devices ,signal generator ,touch-tone telephone ,musical instrument and radio/television transmitter and etc.

4. Oscillators Principles An Oscillator is a type of feedback amplifier in which part of the output is feedback to the input via a feedback circuit. If the signal feedback is of proper magnitude and phase, the circuit produces alternating currents or voltages. The feedback is positive because most oscillators use positive feedback .Finally the closed-loop gain of the amplifier is denoted by rather than . In the Block diagram = + = = B Using this relationship the following equation is obtained: =  

5. However = 0 and 0 implies tha = 1 Expressed in polar form, B = 1/0 or 360 This gives the requirement for oscillations: 1. The magnitude of the loop gain B must be at least 1. 2. The total phase shift of the loop gain B must be equal to 0 or 360 If the amplifier causes phase shift of 180 , the feedback circuit must be provided an additional phase shift of 180 so that the total phase around the loop is 360 . The type of waveform is sinusoidal and are used to illustrate circuit action. The type of waveform generated by an oscillator depends on the components in the circuit and hence may be sinusoidal,square or triangular. In addition ,the frequency of oscillation is determined by the components in the feedback circuit.  

6. Block Diagram

7. Oscillators Conditions A β less than 1 A β Vin is less than Vin the output signal will die out (Damped Oscillation). A β is greater than Vin the output signal will build up . A β is equal to Vin the output signal will steady undamped Oscillations.

8. Frequency Stability Oscillators ability to maintain constant frequency, for as long a period as possible . Frequency depends on : Temperature Power Supply Circuit components Stray elements Supply voltage Active device characterstics But largely frequency depend on RC and LC values

9. Types of Oscillators LC Hartley Colpitt’s Crystal Amstrong RC Phase Shift Oscillators Wein Bridge Oscillators Unijunction or Relaxation oscillators

10. Phase Shift Oscillators The Amplifier (common emitter or inverting amplifier ) produce 180◦ of phase shift because the signal drives the inverting output. Then the RC lag or lead circuit feedback network must produce an additional phase shift network of 180◦ to get a loop phase shift of 360◦ or 0◦. The total phase shift of three RC lag or lead circuits must be equal to 180◦ (approximately 60◦ each). But the RC feedback also produces a significant loss of gain due to the nature of RC network .So the amplifier must produce sufficient gain to overcome this loss at the frequency of operations.

11. = =   At this frequency ,the gain must be at least 29.that is =29 =29 The circuit will produce a sinusoidal waveform of frequency if the gain is 29 and the total phase shift around the circuits is exactly 360  

12. Wein Bridge Oscillators It generates an oscillatory output signal without having any input source. The Wien-bridge oscillator is a unique circuit because it generates an oscillatory output signal without having a sinusoidal input source. Instead, it uses capacitors with initial voltages to create the output. This circuit can be especially useful if connected to a voltage follower to de-couple the load from the source. the Wien-bridge oscillator uses two RC networks connected to the positive terminal to form a frequency selective feedback network. It also amplifies the signal with the two negative feedback resistors.

13. Circuit diagram of wein bridge oscillator The phase angle criterion for oscillation is that the total phase shift around the circuit must be 0 . This condition occurs only when the bridge is balanced.that is at resonance . The frequency of oscillation is exactly the resonant frequency of the balanced wein bridge and is given by = = Assuming that the resistors are equal in value ,and the capacitors are equal in value in the reactive leg of the wein bridge ,at this frequency the gain required for sustained oscillation is given by = =3 1+ = 3 =2  

14. Wein Bridge Oscillator Advantages of Wein bridge oscillators The frequency of oscillation can be easily varied just by changing RC network . High gain due to two –stage Stability is high. Disadvantage of Wein bridge Oscillators The main disadvantage of the Wein –bridge oscillators is that a high frequency os oscillation cannot be generated.

15. RC Oscillators

16. Comparison between RC phase shift and wein bridge oscillators RC phase shift Wein Bridge Used for the low range Used for the low range Feedback is with RC sections. Feedback is lead –lag network Feedback network introduce 180◦ phase shift. Feedback network does not introduce any phase shift. Op-amp is used in inverting mode Op-amp is used in non-inverting mode. Amplifier gain is | A| 29 Amplifier gain is | A| 3 Frequency of the oscillation is = Frequency of the oscillation is = RC phase shift Wein Bridge Used for the low range Used for the low range Feedback is with RC sections. Feedback is lead –lag network Feedback network introduce 180◦ phase shift. Feedback network does not introduce any phase shift. Op-amp is used in inverting mode Op-amp is used in non-inverting mode.