FIELD OSCILLATOR
Since the ramp-generator function is performed by a capacitor- charging circuit, the only requirements of the field oscillator are that
(a) it closes an electronic switch once per 20 ms to discharge the ramp capacitor; and (b) that its free-running frequency be just below 50 Hz to enable it to be synchronised, or triggered, by incoming sync pulses. Many oscillator configurations are possible, though the oscillator must run free in the absence of sync pulses to prevent damage to the picture-tube’s screen when tuning, or during breaks in transmission. All receivers use IC-based field oscillators, in which a form of multi- vibrator (astable) oscillator is most commonly used. Where a field hold control is provided this adjusts the time-constant of the RC timing network.
An alternative technology is to count line synchronising pulses, trig- gering the field flyback after 312^ of them. A representative system will be described later in this chapter.
FIELD OUTPUT STAGE
Even though it is most often incorporated inside an IC, the most common configuration for a field output stage in current practice is the class B type, in which a pair of transistors are connected in series across the d.c. supply line with the scan-coil load connected (via a d.c. blocking capacitor) to their mid-point. The circuit design is similar to that of an audio output amplifier, with both transistor bases being driven together by the sawtooth; each output transistor conducts for half the scanning stroke, the crossover point taking place at screen centre. A typical circuit (simplified) is shown in Fig. 10.2, where the incoming sawtooth waveform comes to TR1 base via C1 and R1. TR1 collector load consists of split resistor R4/R5, across which appears an amplified sawtooth for application to the commoned bases of complementary-symmetrical output pair TR2/TR3.
At the commencement of scan TR1 collector voltage is high, and TR2 fully conductive as a result. As forward scan progresses TR2 turns gradually off, reducing current in the scan coil via R7, C4 and low-value resistor R11. At the mid-point of scan TR2 is almost off, and TR3 beginning to be driven into conduction – a smooth changeo- ver is ensured by the base-voltage offset introduced by preset R2 and temperature-compensating thermistor R3. For the second half of field scan TR3 is driven progressively harder into conduction by the falling ramp at TR1 collector, to the point where the former is almost saturated at scan-end.
Flyback is initiated by a sharp drop in drive voltage at TR1 base, rapidly turning it and TR3 off. The upper plate of scan-coil coupler C4 is at almost ground potential, and this large capacitor cannot quickly charge. TR2 saturates and D1 conducts, clamping the top end of the scan coils to HT potential at decoupler C2. The full sup- ply voltage is now present across the scan coils, whose magnetic field rapidly reverses (t flyback = 1 ms) as a result. The bootstrap capacitor C3 ensures that TR2 remains on and TR3 off during flyback; the same capacitor applies positive feedback to the tap on TR1 collector load during forward scan, increasing the circuit efficiency.
The low-value sampling resistor R11 develops a sawtooth voltage proportional to yoke current, passed via R9 as a.c. negative feedback
to TR1 base to improve the scanning linearity. A second feedback path, this time with d.c. continuity, comes from the lower end of the scan coils to TR1 base via preset R10. Its purpose is to stabilise the mid-point voltage of the output stage, permitting the output voltage swing to be symmetrical between supply rail and ground. Any rise in mid-point voltage increases conduction in TR1, which pulls down the base voltage of the output pair (and hence their emitters’ volt- age) to compensate. Balance is set by adjustment of R10. The second preset R2 sets a small standing (quiescent) current in the output pair to avoid crossover distortion at the point where TR2 hands over to TR3.
The circuit described above is but one variant of many which have been used as field output stages. In most cases the class B output stage is incorporated in an IC which directly drives the yoke via a coupling capacitor; some large-screen receivers and monitors use a class B ‘power booster’ downstream of the IC’s output stage. For camera viewfinder applications a small IC is adequate for the low energy requirement.