GB2186138A - Television driver circuit - Google Patents

Abstract

Colour television driver circuit 1 includes input terminals 2, and input coupling capacitor 3, an input- switching transistor 4 with a biasing resistor 5 and a complementary pair of transistors 6 and 7, whose bases are connected directly together and whose emitters are connected directly together and to a driver output transistor 11 via a limiter inductor 12 and a storage capacitor 13. At the start of the fly back period, a pulse causes input transistor 4 to conduct so that output transistor 11 has no base current supply and so is off. Once the flyback period finishes, the pulse is removed from input terminals 2, so that 7 current now flows through capacitor 13 and turns on transistor 11, building up a charge in capacitor 13, the magnitude of current flow limited by resistor 10. At the end of the scan period, transistor 4 receives another pulse, capacitor 13 discharges its contents forcing the base of transistor 11 to go negative thereby producing the required reverse base current to turn off transistor 11. <IMAGE>

Description

SPECIFICATION Television driver circuit The present invention relates to a horizontal driver circuit for use in television receivers.

In a horizontal driver stage for a colour television receiver, it is necessary to form appropriate current signal shapes for feeding to the output drive transistor, so as to produce a sawtooth deflection current in the line deflection coil with adequate voltage. Conventionally, a transformer is used to achieve current signals of appropriate shape and adequate size for this output drive transistor.

However, such-a transformer is a relatively expensive element of the driver circuit and its use is comparatively in efficient because of leakage inductance losses inherent to the operation of the transformer.

Thus a form of transformer-less drive circuit was proposed in the patent specification accompanying German Patent No. 3041835.

This drive circuit has two complementary transistors whose bases are interconnected by a capacitor and whose collectors are directly connected together; a line-frequency switching voltage is applied to the bases and the output signal at the collectors is fed to a driver output transistor. Thus the specific arrangement of complementary transistors replaces the transformer of the earlier driver circuit.

However, in order to protect this arrangement from the risk of both transistors conducting in the intermediate point of the drive cycle, the base-interconnecting capacitor acts such as to round-off the flanks of the switching voltage. The resultant slow-switching edge is highly detrimental to the efficient and effective operation of the drive output transistor.

An object of the present invention is to provide a transformer-less drive circuit which omits these disadvantages of the prior art.

The present invention provides a driver circuit for a television receiver, the circuit comprising two transistors in complementary, parallel relationship to receive a signal input to the circuit, the complementary transistors being arranged such that the emitters are connected together to provide a signal for feeding to a driver output transistor of the circuit.

Thus, between the switch-off of one of the complementary transistors and the switch-on of the other, there exists an appreciable interval when both transistors are switched off, there existing hysteresis twice the base-emitter voltage in magnitude. Accordingly it is not necessary to interconnect the inputs of the transistors via a capacitor, and hence fast switching can be achieved readily and safely.

Preferably, the collector of one of the complementary transistors is connected to earth while the other collector is connected to a high-tension point, the bases being connected directly together.

Preferably, the driver circuit includes one or more resistance components connected to the collector of the NPN transistor in the complementary transistor pair. Accordingly, the base current can be accurately controlled during the drive period.

The present invention also embodies a television receiver including a drive circuit as defined herein.

In order that the invention may more readily be understood, a description is now given, by way of example only, reference being made to the accompanying drawing in which: Figure 1 is a diagram of a driver circuit embodying the present invention; and Figure 2 is a diagram of waveforms produced in the circuit of Fig. 1.

There is shown in Fig. 1 the circuit diagram for a driver circuit used in a colour television receiver to provide the current regeneration for the sawtooth deflection.

Driver circuit 1 includes input terminals 2, and input coupling capacitor 3, an inputswitching transistor 4 with a biasing resistor 5 and a complementary pair of transistors 6 and 7 in parallel. The bases of the transistors 6 and 7 are connected directly together, the collector of transistor 6 being connected to the earth rail 8, while the collector of transistor 7 is connected to HT point 9 via adjusting resistor 10. The emitters of transistors 6 and 7 are connected directly together and to a driver output transistor 11 via a limiter inductor 12 and a storage capacitor 13. There are also provided a biasing resistor 14 for transistor 7 and diodes 1 5 and 16 to protect the emitterbase junction of transistor 11 from reverse breakdown.

In operation, voltage pulses at line frequency and originating from the osillator are applied to input terminals 2 of driver circuit 1. Resistor 5 normally biases the base such that transistor 4 is off; however the arrival of a pulse at input terminal 2 (i.e. at the start of the fly back period) causes input transistor 4 to conduct. Under this condition transistor 7 is fully off and transistor 6 is fully on, resulting in output transistor 11 having no base current supply and so being off.

Once the flyback period finishes (i.e. when the horizontal scan period starts), the pulse is removed from input terminals 2 whereupon transistor 4 ceases conduction and transistor 7 conducts after finding base bias via resistor 14, resistor 10 enabling complete saturation of transistor 7. Transistor 6 is now fully off.

Current now flows through capacitor 13 and turns on transistor 11, building up a charge in capacitor 13, the magnitude of current flow limited by resistor 10.

At the end of the scan period, transistor 4 receives another pulse, forcing it to conduct and robbing transistor 7 of its base current so that transistor 6 conducts once again. Accordingly, capacitor 13 (previously connected to the HT point 9) is now switched to earth making it discharge its contents accummulated while transistor 11 was switched on. This discharge forces the base of transistor 11 to go negative thereby producing the required reverse base current to turn off transistor 11.

Fig. 2 illustrates the forms of signal produced at transistor 11 by the complementary transistors 6 and 7 as operated in the way described above, ts marking the end of the horizontal scan period and t, marking the end of the subsequent flyback period.

Claims (5)

1. A driver circuit for a television receiver, the circuit comprising two transistors in complementary, parallel relationship to receive a signal input to the circuit, the complementary transistors being arranged such that the emitters are connected together to provide a signal for feeding to a driver output transistor of the current of the circuit.

2. A driver circuit according to Claim 1, wherein the collector of one of the complementary transistors is connected to earth while the other collector is connected to a high-tension point, the bases being connected directly together.

3. A driver circuit according to Claim 1 or Claim 3, wherein the driver circuit includes one or more resistive components connected to the collector of one of the complementary transistors.

4. A driver circuit substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

5. A television receiver incorporating a driver circuit according to anyone of Claim 1 to 4.