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. 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.

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Hartley Oscillator

The Hartley Oscillator

The main disadvantages of the basic LC Oscillator circuit we looked at in the previous tutorial is that they have no means of controlling the amplitude of the oscillations and also, it is difficult to tune the oscillator to the required frequency. If the cumulative electromagnetic coupling between L1 and L2 is too small there would be insufficient feedback and the oscillations would eventually die away to zero. Likewise if the feedback was too strong the oscillations would continue to increase in amplitude until they were limited by the circuit conditions producing signal distortion. So it becomes very difficult to "tune" the oscillator.
However, it is possible to feed back exactly the right amount of voltage for constant amplitude oscillations. If we feed back more than is necessary the amplitude of the oscillations can be controlled by biasing the amplifier in such a way that if the oscillations increase in amplitude, the bias is increased and the gain of the amplifier is reduced. If the amplitude of the oscillations decreases the bias decreases and the gain of the amplifier increases, thus increasing the feedback. In this way the amplitude of the oscillations are kept constant using a process known as Automatic Base Bias.
One big advantage of automatic base bias in a voltage controlled oscillator, is that the oscillator can be made more efficient by providing a Class-B bias or even a Class-C bias condition of the transistor. This has the advantage that the collector current only flows during part of the oscillation cycle so the quiescent collector current is very small. Then this "self-tuning" base oscillator circuit forms one of the most common types of LC parallel resonant feedback oscillator configurations called the Hartley Oscillator circuit.



Basic Hartley Oscillator Circuit


When the circuit is oscillating, the voltage at point X (collector), relative to point Y (emitter), is 180o out-of-phase with the voltage at point Z (base) relative to point Y. At the frequency of oscillation, the impedance of the Collector load is resistive and an increase in Base voltage causes a decrease in the Collector voltage. Then there is a 180o phase change in the voltage between the Base and Collector and this along with the original 180o phase shift in the feedback loop provides the correct phase relationship of positive feedback for oscillations to be maintained.
The amount of feedback depends upon the position of the "tapping point" of the inductor. If this is moved nearer to the collector the amount of feedback is increased, but the output taken between the Collector and earth is reduced and vice versa. Resistors, R1 and R2 provide the usual stabilizing DC bias for the transistor in the normal manner while the capacitors act as DC-blocking capacitors.


Shunt-fed Hartley Oscillator Circuit



In the Shunt-fed Hartley Oscillator both the AC and DC components of the Collector current have separate paths around the circuit. Since the DC component is blocked by the capacitor, C2 no DC flows through the inductive coil, L and less power is wasted in the tuned circuit. The Radio Frequency Coil (RFC), L2 is an RF choke which has a high reactance at the frequency of oscillations so that most of the RF current is applied to the LC tuning tank circuit via capacitor, C2 as the DC component passes through L2 to the power supply. A resistor could be used in place of the RFC coil, L2 but the efficiency would be less.


Hartley Oscillator using an Op-amp

As well as using a bipolar junction transistor (BJT) as the amplifiers active stage of the Hartley oscillator, we can also use either a field effect transistor, (FET) or an operational amplifier, (op-amp). The operation of an Op-amp Hartley Oscillator is exactly the same as for the transistorised version with the frequency of operation calculated in the same manner. Consider the circuit below.

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