Patent GB-1377143-A (GB)
Title: Audio amplifiers
IPC: H03F3/187
Filed: 1972-05-25
Abstract: 1377143 Transistor amplifiers; radio signalling MOTOROLA Inc 25 May 1972 [9 June 1971] 24781/72 Headings H3T and H4L  An amplifier comprises a pair of transistors 50, 51, Fig. 3, with a current source, i.e. transistor 55, as common emitter impedance; each of the transistors 50, 51 is followed by a two-stage amplifier with D.C. feedback to the corresponding transistor. The two stage amplifiers comprise transistors 58, 59 and 61, 62 respectively, the transistors 59, 62, comprising a class B output stage driving t ansducer 60. The D.C. feedback is provided by means of auxiliary transistors 64, 72 with their collectors coupled to the bases of transistors 50, 51 respectively over resistive networks, e.g. 66, 67, 68, 70 with a capacitor 69 which removes any A.C. feedback. The base bias of the constant-current transistor 55 is derived from a further D.C. feedback loop comprising transistor 81 and emitter-coupled transistors 80, 78, capacitor 79 removing any A.C. feedback. The amplifier comprises an integrated-circuit chip 30 which is mounted in a thick-film circuit which also includes a preamplifier 45 comprising transistors 46, 48, in which the gain can be given two alternative values by means of an external switch 44, by means of which resistor 43 can be connected in parallel with collector circuit resistor 42. Calling arrangements.-The amplifier is used in a miniature pager (Fig. 1, not shown): it forms the amplifier 20, Fig. 2, in a radio receiver in which calling tones are extracted from the detector circuit 18 by means of tone filters 25 coupled to a decoder 26 coupled to an alert tone oscillator 28. The receiver also comprises a squelch circuit 24. The detector is coupled via terminal 35, Fig. 3 to the base of transistor 46, while the alert tone oscillator is coupled to the base of transistor 48 via terminal 40 and resistor 41; during the alert period the transistor 46 is biased non-conductive by means of a D.C. bias applied to terminal 38. The amplifier obtains its supply via a voltage regulator circuit comprising transistor 85, with emitter connected to the battery and collector to line 86, and a shunt train of transistors 88-91. In the absence of any signal, transistor 94 is conductive, causing the base of transistor 88 to be earthed and thereby rendering transistor 85 non-conductive. During an alert period the D.C. bias on terminal 38 is also applied, via resistors 96, 97, to the base of a transistor 98, rendering it conductive: transistor 94 is thus biased non-conductive whereby the voltage regulator circuit is rendered effective to supply power to the amplifier. When this occurs, a bias is applied over resistor 99 to the base of transistor 100 whose collector circuit shunts that of transistor 98, thereby latching the amplifier operative. Release of the latching action is performed by the squelch circuit: when a carrier signal is received a positive voltage is applied to terminal 10 which turns on transistors 120, 121 thereby biasing transistor 125 non-conductive and enable the latching bias to render transistors 100 conductive. On termination of the carrier transistors 120, 121 become non-conductive, transistor 125 is rendered conductive and so the latching transistors 100 becomes non-conductive : capacitor 135 provides a time-delay. The amplifier may be turned on and off manually by means of switch 128, closure of which applies a bias to transistor 130 which renders it non-conductive: this biases transistor 132 conductive, whereby transistor 94 becomes non-conductive so that the voltage regulator circuit applies power to the amplifier. If switch is opened again immediately, the receiver stays on by the latching action; if, however, the switch is left closed an appreciable time, capacitor 135 charges up and on opening of the switch 128 transistor 125 is turned on, releasing the latching transistor 100 and so turning off the amplifier.