Field output stage
The field output amplifier is made up of a class B complementary- symmetry transistor pair whose mid-point feeds the deflection coils via a large d.c.-blocking electrolytic coupling capacitor.
A typical class B field output stage is shown in Figure 13.19, in which VT8/VT9 is the driver combination and VT10/VT11 is the complementary output pair. At the start of the scan, VT8 base voltage is low, ‘turning off’ VT8 and VT9. VT9 collector is almost at d.c. supply potential turning VT10 fully on, driving current into the field scan coils. VT11 is off. At the mid- point of the scan, the centre of the picture, VT8/9 begins to conduct, turn- ing VT10 off and VT11 on; this provides the scan current for the second half of the picture. W1/W2 provides a small forward bias for the output transistors to prevent cross-over distortion. Resistor R4 provides d.c. feed- back for bias stability.
A more advanced field output amplifier employs a switched-mode modulator (Figure 13.20) in which a pulse train is fed into an LC low-pass filter. As can be seen, the pulses are smoothed with the capacitor charging up to the mean value of the input pulse. Since the mean value is deter- mined by the mark-to-space ratio of the input pulse, a varying mark- to-space ratio produces a varying charge across the capacitor. A linearly
ncreasing capacitor voltage, i.e. a timebase ramp scan, may thus be pro- duced if the mark-to-space ratio of the input pulse is gradually increased. The PWM waveform is first fed into the output stage to turn the active device on and off before going into an LC low-pass filter, which produces the sawtooth waveform. The output device, which may be a transistor or a thyristor, thus acts as a switch. The clock pulse which drives the PWM may be a separately generated waveform or it may be derived from the line scanning pulse. Since the active element is a switch, its power dissi- pation is extremely low. When the switch is on, its resistance is very small and therefore its power dissipation is very low. When the switch is open, current ceases and power dissipation is nil.