المتطلبات الخاصة لدارة الانحراف الراسي الترانزستورية في التلفزيون الأبيض والأسود

15-3 Special requirements for transistor vertical deviation circuit in black and white TV:

The transistor vertical output circuit is similar to the similar fuse circuit: but the difference is in the values ​​of efforts and currents in the elements of each circuit. Valves require greater efforts and smaller current intensity - while transistors require less effort and larger currents - and the large current makes the transistor need some way for thermal stability - it also needs variable tuning resistors, to get a correct operation in the output circuit, especially when we have to replace the transistor during reform process.

Vertical scan line:

To obtain vertical scanning linearity, the current of the deflection coils must rise at a constant rate and with a change of not more than 10% - this is achieved when the output transistor acts as a linear amplifier of class A for the sawtooth wave in the vertical deviation circuit. It reduces the bias of the amplifier in the amplifier by keeping the frequency response lower by (1 Hz) - and the direct coupling method is used wherever possible - and the wave checker and negative feedback are used extensively in vertical deviation circuits. Note (Class A in operation of the transistor is that both the negative and positive peaks of the output waveform are not clipped).

Transistor overheating protection:

Figure (4-15) shows the simplified diagram of the vertical output stage and the thermal protection method for the output transistor. It is a power transistor, and since its current is large in the collector, its temperature rises quickly, especially in the collector junction - and despite the use of a thermal radiator for it, a larger current will pass through the collector and increase the collector-radiator current - and to prevent thermal damage to the transistor - it stabilizes the radiating base-bias current By means of a thermal resistance (thermistor). And it is connected to the base of the transistor: as in the figure.

An increase in the collector current and the temperature cause an increase in the radiating base current, and so a larger current passes through the thermistor, so its temperature rises, and this leads to a decrease in its resistance. The forward bias voltage of the transistor decreases. This, in turn, prevents the collector current from increasing and stabilizes the operation of the amplifier. Additional heat protection is used by the Radiator Resistance RE. Any increase in the collector current causes an increase in the voltage drop on the RE. This leads to a decrease in the forward bias of the transistor. It prevents the collector current from increasing.

Bias Control:

Figure (5-15) shows two resistors to manually control the bias in the vertical output circuit, and the transistors in the vertical output stage require significant bias. To achieve this bias, there are variable voltage segments in the base circuit of the transistor. The resistors R1, R2, R1 are connected in series from the base to the supply voltage VCC - and adjusting the resistance R ensures the correct bias of the transistor. The variable resistance connected from the collector to the base of the transistor (0) provides additional feedback, and makes the possibility of replacing the transistor more tolerant.