Patent GB-1319288-A (GB)
Title: Method of and apparatus for processing an amplitude varying signal
IPC: H03F3/217
Filed: 1970-07-15
Abstract: 1319288 Pulse modulated inverters; control of AC and DC motors ROHR INDUSTRIES Inc 15 July 1970 [14 Nov 1969] 34334/70 Headings H2F and H2J [Also in Divisions H3 and H4] A method of processing an amplitude varying signal includes the steps of forming a pulse width modulated signal from the amplitude varying signal, separating each pulse width modulated signal in approximately half with respect to the time duration of the pulse and passing successive approximate halves of each pulse of the pulse-width modulated signal through separate devices, reassembling corresponding approximate halves of each pulse of said pulse width modulated signal and converting the reassembled pulse width modulated signal to an amplitude varying signal. The term 'approximately half' is defined as including any division from ¢: ¢ up to # : #. In Fig. 1 a signal such as from a microphone audio generator or function generator 1 is fed via a dual transistor 2, comprising two transistors formed on a single chip and in the same metal can, to an integrated circuit comparator 3 which acts like a Schmitt trigger. The dual transistor 2 acts to reduce the effect of temperature drift. Oscillations from a unijunction transistor 5 are frequency divided in a bi-stable 7 which has one of its square-wave outputs fed via an integrator 8 to produce a sawtooth output which is applied to the input so as to provide a pulse width modulated antiphase pulse outputs at 19 and 20. Resistor 21 and capacitor 22 provide the required delay so as to match the delay provided by the integrator 8 and by the positive feedback around comparator 3 via 29 so as to provide antiphase square wave output pulses at 23, 24. The pulse outputs at 19 and 20 co-operate with those from 23, 24 to control NAND gates 39a, b, c, d (Fig. 2). The first half of the pulse on line 19 passes through gate 39a and the second half of the pulse passes through gate 39b. The output from gate 39a operates transistors 40, 41 and transformer 42 to make the transistor 49 pass current until capacitor 50 is charged. The current passing through transistor 49 results in a reverse current from the base of transistor 68 to aid in turning this transistor off. The positive potential supplied by diode 44 now passes via L.P.F. 45, 46, 47 to turn on output transistor 67. The second half of the pulse on line 19 also maintains transistor 67 on due to the gate 39b, transistors 51, 52, transformer 53 and diode 55. The gates 39c and 39d operate in a similar manner for the first and second halves of the pulse on line 20 so as to turn off transistor 67 and turn on transistor 68. The filtering effect of reactor 71 reproduces the original input signal wave. Each power transistor 67, 68 may consist of a number of transistors in parallel with a corresponding number of diodes 89, 90. The diodes 44, 55 may each be four glass diodes connected in parallel. The amplifier forms a Class D amplifier. The elements 4a, 4b, 4c, 4d and 4e may be formed as an integrated circuit as may be the integrator 8. The transistors used may be Si, Ge, or Ga As or may be F.E.T. The gates 39 may alternatively be OR, NOR or AND gates and the DC power supplies may be fuel cells. The half bridge circuit of Fig. 2 can be modified to a full bridge (Fig. 4, not shown) including two additional power transistors (67a, 68a). This may be further modified (Fig. 5, not shown) by having two input circuits (I, Ia) of Fig. 1 fed in antiphase by including an input transistor (93) and the output of each of these input circuits (I, Ia) feeds a respective switching circuit (Mg, Mag) similar to Fig. 2. The outputs of these switching circuits (Mg Mag) is appropriately connected to the output transistor bridge circuit (of Fig. 4). In a further modification (Fig. 6, not shown) transistor 68 and associated drive circuitry is omitted and such a circuit may be used to supply power to a D.C. motor. Also by having a circuit including Figs. 1 and 2 for each phase, two and three phase circuits can be provided (Figs 7 and 8, not shown). By varying the oscillator frequency speed control of an induction or synchronous motor load 72, such as for a drill press, may be obtained. The output may feed loudspeakers including those used underwater.