LC feedback Oscillator
These are resonant circuit or tank circuit oscillators. They are commonly use to produced high frequencies as ranging from I MHz to 500 MHz Hence they are also known as RF oscillators. These oscillators are used in RF generators, radio transmitters and receivers. Some popular IC feedback oscillators are of the following types.
- Hartley Oscillator
- Tuned-collector of Armstrong Oscillator
- Colpitts Oscillator
Hartley oscillator is very popular and is commonly used as a local oscillator in radio receivers. The main advantages of this oscillator is wide range of frequencies and easy to use.
Figure shows the circuit of Hartley oscillator. Its tank circuit consists of two coils L1 and L2. The coil L1 is inductively coupled to coil L2 and the combination works as an auto transformer. A coil called radio frequency choke RFC is connected between the collector and the Vcc supply. It acts as load for the collector and also permits an early flow of D.C current but blocks AC current. The feedback between the output and input circuit is accomplished, through auto transformer actions which also introduces a phase shift of 180o. The phase reversal between the output and input voltage occurs because they are taken from the opposite end of the coil L1 and L2 with respect to the tap, which is grounded. It may be observed that the tap on the combination L1 and L2 coils is actually connected to the transistor emitter terminal via ground and through the capacitor CE. As the transistor also introduces a phase shill of 180°, therefore the total phase shift is 360° and hence the feedback is positive.
The frequency of the oscillation is given by the relation
As the capacitor Cc connected between the collector and the tuned circuit permits only the A.C currents to pass to the tank circuit. It means capacitor Cc blocks the D.C current. The capacitor CB is called blocking capacitor it further blocks the D.C current from reaching to the base. The resistors R1, R2 and RE are used to provide D.C bias to the transistor.
The colpitts oscillator is widely used in commercial signal generators from frequencies between 1 MHz and 500 MHz. It is similar to the Hartley oscillator with minor modifications. Instead of using a tapped inductance, two capacitors are tapped as shown in Figure. The frequency of oscillations if mutual inductance ignored is given by the relation
As the points A and B are 180° out of phase with each other. Also another phase shift of 180° is provided by the transistor itself hence there is a total phase shift of 360°between the emitter-base and collector-base circuits. When the supply is ON, the capacitors C1 and C2 are charged. These capacitors discharge through the coil L, which set up the feedback to the base-emitter junction and obtain in an amplified form at the collector of the transistor. Due to the positive feedback, the oscillation of constant amplitude is obtained.
Tuned Collector (Armstrong) Oscillator
A tuned collector oscillator circuit using a transformer is shown in Figure. The primary of the transformer forms a tuned circuit with capacitor C and it decides the frequency of oscillation. Its amplifier provides a phase difference of 180° and an additional phase difference of 180° is provided by the transformer, it will result in positive feedback. For the oscillatory action, the transistor amplifier provides sufficient gain. The resistors R1, R2 and RE provide D.C bias to the transistor. CE and C2 are bypass capacitors, so that resistors RE and R2 have no effect on A.C operation. The D.C bias is provided by the resistors R1 and R2 through the low-resistance secondary winding which also provides A.C feedback.
The output of the tuned collector oscillator is taken by using transformer coupling. The output terminals are connected to the power consuming device or input terminals of the next stage. This loads the tank circuit and its effective dissipative resistance increases and circuit losses are increased. To maintain oscillations, more positive feedback is required which is achieved by increasing coupling of the transformer. The frequency of oscillations produced by the tank circuit depends upon the value of primary inductance L and capacitance C and its value is given by the relation.