المذبذب الحاجز الترانزستوري .. كتاب التلفزيون الملون والعادي

Chapter Fourteen:

Virtical oscillator and deflection circuits


14-1 Blocking oscillator

The barrier oscillator is used in the horizontal and vertical deviation circuits and we will now explain the vertical oscillator while we will return to the horizontal oscillator and the AFC circuit
(Automatic Frequency Adjuster) in later paragraphs.

The working principle of the valve or transistor barrier oscillator is similar, despite the existence of a method of opening and closing the transistor, which requires a sawtooth wave generated in the transistor circuit itself. We often find that transistor deviation circuits include one or more stages of driving (Driver), between the oscillator and the power amplifier, and this is due to the inability of the oscillator output to operate the output circuit completely - and the direct connection method between the oscillator circuit and the output amplifier circuit may be used in devices with a screen Small dimensions, and when using an oscillator circuit with sufficient power, then the output of the amplifier can generate the current necessary to operate the deflection coils.

Typical circuit of a barrier oscillator and a vertical path generator:

Tightly connected transformer windings provide positive feedback from the collector to the base of transistor Q1. The transistor is of class C. And it is in the state of connection for a short period. The RC circuit, which is R2-C2, performs two tasks: determining the frequency of the oscillator and generating a wave from the saw (Fig. 14-1). When the supply voltage is applied, the circuit works as follows: A negative bias voltage appears on the base of the Q1 transistor (PNP) resulting from the voltage divider R1- R3 - The capacitor C2 is not charging.
A - transistor Q1 in the conductive state:

When Q1 is connected, C2 begins to charge in the negative direction and the collector current increases in the primary coils of the transformer T1 and voltage arises in the secondary coils with the polarity shown in the figure.

The base voltage consists of two series voltages with earth: the voltage across R1 and the secondary voltage of T1. And both efforts help together and push the base to a negative polarity, the largest increase in the forward bias. Thus, transistor Q1 opens more strongly and capacitor C2 charges rapidly and linearly. Creates the return part of a wave from the saw.

B - the transistor in the cut-off state:

In the regeneration cycle in which C2 continues to charge to the highest negative voltage, until Q1 reaches saturation, where this saturation causes the collector current level to be in the saturation level, and the transformer T1 has the strong and constant magnetic field in the primary coils and no voltage is generated in the secondary coils For the transformer at the moment of collector current saturation (voltage is generated only in the transformer when a change in current occurs), the base voltage of Q1 drops to the voltage in the resistor R1 - at this time the voltage of C2 (reverse bias) exceeds the base voltage (forward bias). The transistor cuts off. 01

Line generation:

The transistor is now in the cut-off state - capacitor C2 starts to discharge through R2 generating the sawtooth line up and in the positive direction - and 01 stays cut-off until the voltage on R2-C2 drops to the same value as the forward bias voltage on R1. When this happens, the base junction becomes the emitter again with forward bias, the transistor connects, and the charge cycle begins again - and the slow discharge of the capacitor C2 in R2 produces the upside of the sawtooth and its timing

Synchronization: The synchronization process is done by applying negative synchronous pulses to the base, and as in the diaphragm oscillator - the natural frequency of the oscillator must be slightly less than the frequency of the synchronization pulses, as the synchronization pulses force the transistor to be connected just before the C2 vacuum

Self-oscillation: The frequency of the oscillator is controlled by C2-R2 in the emitter circuit - and it is noted that during the discharge the transistor Q1 is cut off and the C2-R2 circuit is isolated from the rest of the circuit. This helps to secure the stability of the oscillator frequency, and the natural frequency can be slightly biased by the DC change of the forward bias (on R1) applied to the secondary windings of the transformer. And the resistance R3 is part of the network bias, and achieves the task of adjusting stability, and increasing the negative voltage across R1 (increasing the forward bias) allows the Q1 transistor to stop connecting immediately. Thus, the base natural frequency increases, via .

Transient Elimination: During the transistor cut-off time, the voltage of the transient pulses appears across the primary and secondary of the transformer T1. And it could disrupt the work of the circuit. Because it can exceed the collector breakdown voltage - the base of the transistor Q1 - and most of the transient waves can be removed by connecting a binary through the primary coils as in Figure (14-1) - the binary is connected when the polarity of the voltage of the transformer T1 reflects what is seen, and then the power of the magnetic field disperses It is harmless in the binary, but it came out of the saw applied to the stage of the vertical stimulus.