Harmonic Oscillators

Some Harmonic oscillators ppt for those who need it...........

1. Presentation On Harmonic Oscillators By:- Vishal Thakur

2. What is an oscillator…??? An oscillator  is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave. They are widely used in electronic devices .

3. Harmonic Oscillators The harmonic, or  linear , oscillators are those oscillators that produces a sinusoidal output . The basic form of a harmonic oscillator is an electronic amplifier connected in a positive feedback loop  with its output fed back into its input through a filter .  

4. Essential Parts of any Oscillator Tank Circuit (R-C , L-C , etc.) Transistor Amplifier (180 ° phase shift) Feedback Circuit

5. Barkhausen Stability Criteria The loop gain should be unity. i.e A β = 1 Phase shift around loop is zero or an integral multiple of 2 π (Positive Feedback).

6. Tuned Collector Oscillator It is called the tuned-collector oscilla­tor because the tuned circuit (L-C circuit) is connected to the collector of the transistor. The tuned circuit constitutes of the capacitor C and inductor L. Some Harmonic Oscillators…

7. When the supply is switched on, a transient current is produced in the tuned L-C circuit. It induces voltage in L 1  by mutual induction which causes corresponding vari­ations in base current. These variations in base current are amplified β times and appear in the collector circuit. A part of this amplified energy is used to meet the losses that occur in the tank circuit and the rest is radiated out.

8. Frequency of oscillation , f = 1/2 π √ (L.C)

9. Colpitt’s Oscillator In this type of oscillator , the transistor amplifier’s emitter is connected to the junction of capacitors, C 1  and C 2  which are connected in series and act as a simple voltage divider. When the power supply is firstly applied, capacitors  C 1  and C 2  charge up and then discharge through the coil L.

10. The amount of feedback depends on the values of  C 1  and C 2   Feedback Coefficient , β = C 1 /C 2

11. Hartley’s Oscillator W idely used as local oscillator in radio receivers . Hartley oscillator circuit is similar to Colpitt’s oscillator circuit, except that phase-shift network consists of two inductors L 1  and L 2  and a capacitor C instead of two capacitors and one inductor . Feedback Fraction , β = L 1 /L 2

12. F = 1 / 2 π √ [C (L 1  + L 2  + 2M )]

13. Phase Shift Oscillator The phase shift oscillator produces positive feedback by using an inverting amplifier and adding another 180° of phase shift with the three high-pass filter circuits. The most common way of achieving this kind of oscillation is using three identical cascaded resistor-capacitor filters.

14. Wein -Bridge Oscillator It is one of the most popular type of oscillators used in audio frequency ranges. It is a two-stage amplifier with an R-C bridge circuit . This type of oscillator is simple in design, compact in size, and remarkably stable in its frequency output. Furthermore , its output is relatively free from distortion and its fre­quency can be varied easily .

15. The overall gain is high because of use of two transistors . When C 1 = C 2 = C and R 1 = R 2 = R

16. Crystal Oscillator This is a type of oscillator that is controlled by a crystal. The big advantage of a crystal oscillator is high frequency stablility . Common crystals used are Rochelle salts and quartz . The natural frequency of a crystal's vibrations is found to be more constant than the oscillations in a LC circuit . Natural Ferquency = K/T where K = cut angle constant and T = thickness of crystal

17. The LC circuit is the electricial equilavent of the crystal. It has limited power output. Crystals will overheat or crack when fed with too much voltage. The current flowing through a crystal generally should not be more than 100mA (.1A ) Frequencies :- Series resonant Frequency , f s = 1/2 π √(L.C ) Parallel Resonant Frequency

18. Crystal Circuit Equivalent Circuit Quality factor = √L/(R √C)