GB1185311A - Improvements in or relating to Field Time-Base Audio Amplifier Circuits Employing Transistors. - Google Patents

Abstract

1,185,311. Transistor amplifiers; sawtooth current generating circuits; cathode ray tube circuits. MULLARD Ltd. 7 April, 1967 [11 Jan., 1966; 29 June, 1966], No. 1230/66. Headings H3T and H4T. An audio amplifier or sawtooth generator comprises two complementary transistors Tl, T2 connected in series collector to emitter by a resistor R1, an output circuit and a capacitor connected between the emitter of T2 and a supply rail the base of T2 being connected to a drive and bias circuit comprising a potentiometer R2, R3 connected at one end to the collector of T1 and at the other to a point which follows the sawtooth wave across the output circuit and which rises beyond the rail voltage connected to the collector of T2. To generate a sawtooth wave capacitors C1 and C2 are charged through resistor R5 to gradually increase the conduction of T1 At the start of the wave the base voltage of T2 derived from the steady voltage across C4 via potentiometer R3, R2 makes T2 heavily conducting but this voltage is progressively offset by the rise in voltage across R1 due to the rising current in T1 until at the end of the wave T2 is off. A sawtooth current wave is thus generated in the deflection coils Ly. At the end of the scan a switch SW (representing the circuit of Specification 935,722) is closed to discharge the capacitors and render T1 non-conducting. Resonant flyback of the circuit Ly, C3 occurs swinging point A and thus the emitter and base of T2 negatively of the supply, conduction of T2 being blocked by D2 until point A becomes positive of the supply, clamping the coil voltage until the current reversal is completed. Linearity is increased by feedback through Rlin. Modifications.-Diode D2 and the resonating capacitor C3 may be omitted, the flyback then being of square instead of sinusoidal waveform. C4 and Ly may be interchanged in position. In Fig. 5 the field coil is series resonated by a capacitor (Cys) at the line frequency. In addition pin cushion correction may be effective as described in Specification 1,081,676 by adding line pulses to the deflection coil circuit and to the emitter of T1. Conventional convergence waveforms for a colour television receiver are also made available. D.C. shift is effected by a potentiometer (R10, Rs) connected to the deflection coils and to a further supply source. In addition a D.C. driver amplifier To is provided for T1. In Fig. 6 (not shown) D.C. correction is provided by a potentiometer (R14, R15, R16) connected to the terminal of the deflection coil which is at a steady potential. Capacitors (C4a, C4b) serving the purpose of C4 in Fig. 3 are connected between the other end of Ly and respective supply rails. The supply for the upper end of resistor R3 is provided through a coupling capacitor (C7). Conventional convergence waveforms are also available in this circuit. Operation in the so called " Class D mode " may be introduced by including a pulse width or frequency modulator M at the input to T1 as described in Specifications 1,076,092 and 1,076,093. Operation in this mode as an audio frequency amplifier may be obtained by replacing the drive shown by an audio frequency source and the deflection coil by a suitable load.