Patent GB-1266375-A (GB)
IPC: H04M19/02
Filed: 1969-09-12
Abstract: 1,266,375. Electronic A.C. generators; telephone ringing generators. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 12 Sept., 1969 [17 Sept., 1968], No. 45157/69. Headings H3T and H4K. [Also in Division H2]  An A.C. generator comprises a pulse generator A with a variable pulse width and with a fixed frequency which is higher than that of the A.C. to be generated, a switching amplifier B for amplifying the output pulses of the generator A, a low pass filter C connected to the output of the amplifier B and through which the A.C. generated can be supplied to an output D, and a tuned circuit E, included in a regenerative feedback path, tuned to the frequency of the A.C. to be generated and by way of which the generator A can be pulse width modulated by the output voltage of the low pass filter C. The sawtooth output from Miller integrator 1 is fed via amplifier stages 11, 19; 22, 23 and 24, 25 which provides a differentiating effect to produce a square wave output from transformer 31. The generator output is maintained by the regenerative feedback connection via resistor 33 and the pulse to space duration is adjusted to be equal by adjusting resistor 9 and diode 26 also improves this equality. A further regenerative feedback path via resistor 35 improves the steepness of the output pulses. Resistor 21 may be replaced by a diode poled in its pass direction or an inductance. The output from generator A is coupled via transformer 31 to input windings 36-39 of a switching Class D push pull amplifier B in which transistors 40, 40<SP>1</SP>, 43, 43<SP>1</SP> conduct alternately with transistors 41, 411, 42, 42<SP>1</SP> so as to produce pulses of opposite direction from the output winding 63 of transformer 62. A low pass filter C formed of a capacitor 64 and the inductance of the winding 63 acts to suppress all harmonics of this square wave output at 63 such as 25 kHz and allows only a small part of this to pass to the output D. When switch 73 is closed the tuned circuit E is excited by the small ripple at the output D and an alternating voltage at the natural frequency, say 30 Hz, of this circuit is produced across the secondary 68. The output from 68 causes periodic variation of the operating point of transistor 1 so as to pulse width modulate the output pulses from the generator A. An alternating voltage (V D , Fig. 3, not shown), is produced at the output D. The output voltage may be made more sinusoidal (V<SP>1</SP> D ) by diodes 74, 75 which limit the modulation voltage. Further harmonics may be eliminated by a negative feedback connection via 76, 77 which produces a sinusoidal output (V<SP>11</SP>D) at D. The arrangement is protected against short circuits since when the output terminals are overloaded the modulation and production of the low frequency voltage are suppressed. The transformer 31 may be omitted and the transistors 24, 25 may be coupled to the amplifier B as stage 22, 23 is coupled to stage 24, 25. The load connected to D may be inductive such as a synchronous motor. The output A.C. frequency may be adjusted by varying the natural frequency of the tuned circuit 65, 66. In a modification (Fig. 4, not shown), forming a ringing generator for a telephone system the output stage (24<SP>1</SP>, 25<SP>1</SP>) forms the switching amplifier B and a direct current negative feedback loop having a time constant larger than the period of the A.C. output is provided. This negative feedback loop includes a transistor (91) and integrating circuits (95, 96; 97, 98 and 99, 100) and a diode (94) acts to clamp the base of the transistor (91). The voltage across an output capacitor (29) is compared with the voltage across a resistor (92) connected to the base of transistor (91) and this controls the negative feedback so as to stabilize the circuit to maintain the symmetry of the switching impulses generated in the absence of modulation. An additional negative feedback path including a capacitor (87) in series with a resistor (76<SP>1</SP>) and capacitor (77<SP>1</SP>) connected in parallel is provided between the output of generator A and the input of transistor 1. Capacitors (29, 29<SP>1</SP>) connected across the input supply and to the output of the generator A may be used as D.C. sources provided the current drawn through a winding 61 of the output transformer 62 does not saturate the core of this transformer.