14-10-2014, 06:52 PM
. CHAPTER 2: OSCILLATOR
2. 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 criterion is that the signal fed back to the input of the amplifier must be in phase. In-phase feedback is also called positive feedback, or regenerative feedback. It is an unstable amplifier.
3. Electronic oscillators are divided into: Sinusoidal (or harmonic) oscillators-which produce an output having sine waveform Non-sinusoidal (or relaxation) oscillator-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. BLOCK DIAGRAM Closed loop transfer function with positive feedback: If, βA= 1 + j0 or βA = 1∠0o
5. Barkhausen criterion : The feedback factor or loop gain . The gain is infinite, this represent the condition for oscillation. The net phase shift around the loop 0 ˚ (or an ˚ integral multiple of 360 ). In other word, feedback should be positive. The amplifier gain must be greater than the loss in the feedback path.
6. OSCILLATOR CONDITION Aβ less than 1 AβVin is less than Vin the output signal will die out (Damped Oscillation). Aβ greater than 1 AβVin is greater than Vin the output signal will build up. Aβ equal to 1 AβVin is equal to Vin the output signal will steady, Undamped Oscillations (Stable oscillator).
7. TYPE OF OSCILLATOR LC Hartley Colpitt’s Crystal Amstrong RC Phase Shift (RC)
8. HARTLEY OSCILLATOR The configuration of the transistor amplifier is of a Common emitter amplifier with the output signal 180o out of phase with regards to the input signal These two inductances form an autotransformer action and gives the feedback with a phase reversal of 180°, thus the total phase shift becomes 360o to give the feedback positive or regenerative feedback. A Hartley oscillator uses an inductive (single tapped- coil) of L1 and L2. Voltage divider to determine the feedback ratio. If ignore Mutual inductance,
9. When the LC tank is resonant, the circulating current flows through L1 in series with L2. The equivalent L to use in equation is: L T = L1 + L2 +2M or L T ≅ L1 + L2 L1 ,is a primary L2 ,is the secondary M, Mutual inductance between the coils. The tuning capacitor CT allows the Hartley oscillator to be tuned over a wide range of frequency The lowest frequency is determined by the maximum capacitance of CT or otherwise. The frequency is determined by the tank’s resonant frequency: To start Oscillating, the circuit needs a minimum voltage gain or must be greater than 1/β , If ignore Mutual inductance
10. a) FET shunt-Fed Hartleyb)Transistor series-fed Hartley
11. R1 and R2 – provide the usual stabilizing DC bias for the bipolar transistor. C1 and C2 – as a dc- blocking capacitor that provides low impedance at the oscillator’s operating frequency while preventing the transistor’s dc operating point from being disturbed and less power is wasted when DC flow through inductive coil. The radio frequency choke (RFC) - in providing the amplifier with a steady dc supply while eliminating unwanted ac disturbances. RE,CE Fet and RS, CS Bipolar – to improve amplifier stability (temperature effect) and provide ac ground thereby preventing any signal degeneration.
2. 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 criterion is that the signal fed back to the input of the amplifier must be in phase. In-phase feedback is also called positive feedback, or regenerative feedback. It is an unstable amplifier.
3. Electronic oscillators are divided into: Sinusoidal (or harmonic) oscillators-which produce an output having sine waveform Non-sinusoidal (or relaxation) oscillator-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. BLOCK DIAGRAM Closed loop transfer function with positive feedback: If, βA= 1 + j0 or βA = 1∠0o
5. Barkhausen criterion : The feedback factor or loop gain . The gain is infinite, this represent the condition for oscillation. The net phase shift around the loop 0 ˚ (or an ˚ integral multiple of 360 ). In other word, feedback should be positive. The amplifier gain must be greater than the loss in the feedback path.
6. OSCILLATOR CONDITION Aβ less than 1 AβVin is less than Vin the output signal will die out (Damped Oscillation). Aβ greater than 1 AβVin is greater than Vin the output signal will build up. Aβ equal to 1 AβVin is equal to Vin the output signal will steady, Undamped Oscillations (Stable oscillator).
7. TYPE OF OSCILLATOR LC Hartley Colpitt’s Crystal Amstrong RC Phase Shift (RC)
8. HARTLEY OSCILLATOR The configuration of the transistor amplifier is of a Common emitter amplifier with the output signal 180o out of phase with regards to the input signal These two inductances form an autotransformer action and gives the feedback with a phase reversal of 180°, thus the total phase shift becomes 360o to give the feedback positive or regenerative feedback. A Hartley oscillator uses an inductive (single tapped- coil) of L1 and L2. Voltage divider to determine the feedback ratio. If ignore Mutual inductance,
9. When the LC tank is resonant, the circulating current flows through L1 in series with L2. The equivalent L to use in equation is: L T = L1 + L2 +2M or L T ≅ L1 + L2 L1 ,is a primary L2 ,is the secondary M, Mutual inductance between the coils. The tuning capacitor CT allows the Hartley oscillator to be tuned over a wide range of frequency The lowest frequency is determined by the maximum capacitance of CT or otherwise. The frequency is determined by the tank’s resonant frequency: To start Oscillating, the circuit needs a minimum voltage gain or must be greater than 1/β , If ignore Mutual inductance
10. a) FET shunt-Fed Hartleyb)Transistor series-fed Hartley
11. R1 and R2 – provide the usual stabilizing DC bias for the bipolar transistor. C1 and C2 – as a dc- blocking capacitor that provides low impedance at the oscillator’s operating frequency while preventing the transistor’s dc operating point from being disturbed and less power is wasted when DC flow through inductive coil. The radio frequency choke (RFC) - in providing the amplifier with a steady dc supply while eliminating unwanted ac disturbances. RE,CE Fet and RS, CS Bipolar – to improve amplifier stability (temperature effect) and provide ac ground thereby preventing any signal degeneration.
