Tuned Collector Oscillator
The basic circuit of a tuned collector oscillator is shown in figure. It is called the tuned-collector oscillator, because the tuned circuit is connected to the collector. The tuned circuit, constituted by the capacitor C and transformer primary coiL, forms the load impedance and determines the frequency of oscillation. The resistors R1 R2 and RE form the dc biasing circuit of the transistor. Capacitors C1 and CE are bypass capacitors for R9 and RE respectively so that the ac operation of the circuit is not affected. Moreover, C1 provides ac ground for the transformer secondary. The output voltage developed across the tuned circuit is inductively coupled to the base circuit through transformer secondary coil L1. The feedback voltage appears across the base-emitter junction, as the junction point of resistors R1 and R2 is at ac ground due to bypass capacitor C1. Worth noting point is that in the absence of C1, the feeding back voltage induced in the secondary of the transformer would not be directly going to the input of the transistor, some of this voltage will drop across resistor R^TA phase shift of 180° is provided by the transistor amplifier, as it is connected in CE configuration. Another phase shift of 180° is provided by the transformer. Thus a total phase shift of 360° appears between the input and output voltages i.e. there is a positive feedback between the input and output voltages. The transistor amplifier provides sufficient gain for oscillator action to take place.
Working of Tuned Collector Oscillator
When the supply Vcc is first switched on, a transient current is caused in the tuned L-C circuit. It is due to increase of collector current to its quiescent value. This transient current initiates natural oscillations in the tank circuit. These natural oscillations induce some voltage into L1 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 in the form of electro-magnetic waves. The turn-ratio of L and L1 is determined by the total losses. Higher is the turn-ratio, lesser is the feedback voltage applied and vice-versa. The frequency of oscillation, that is, the frequency at which Barkhausen criterion is satisfied differs from the resonant frequency of the tuned circuit. This is due to loading of the transformer secondary to some extent.
Use of Natural Oscillations Setup in Tank Circuit.
The natural oscillations setup in the tank circuit is usually coupled to the next stage in the electronic system. There are various methods of coupling the output of the oscillator to the next stage. These include capacitive, transformer and impedance-coupling networks. Usually the output is taken from the circuit using inductive coupling, as shown in figure. The output terminals are usually connected to the input terminals of the next stage in rare cases to the power consuming device. This has the loading effect on the circuit causing increase in the circuit losses. For maintaining oscillations, more amount of positive feedback, which can be provided by simply increasing the coupling between the primary and secondary of the transformer, is required. In case the load connected across the output terminals of the oscillator is too, large, it may damp the oscillations. This is the reason that in a signal generator the output of the oscillator is connected through a buffer amplifier (emitter follower).