GB1084018A - Apparatus comprising a microphone and an amplifier - Google Patents

Abstract

1,084,018. Capacitor microphones; transistor amplifiers; substation circuits. NORTHERN ELECTRIC CO. Ltd. Sept. 4, 1964 [Sept. 4, 1963], No. 36441/64. Headings H3T, H4J and H4K. A self-polarized electrostatic microphone has its output connected to the matched input of a semi-conductor amplifier. The diaphragm is of polyethylene terephthalate sheet, on one side of which is deposited an aluminium layer, and is prepolarized by heating it to about 120‹ C., exposing it to an electrostatic field having a potential gradient of 20 KV/cm., and allowing it to cool in the field. In one construction (Fig. 4), a perforated or indented metal plate 69 supports the diaphragm and forms one end of a cylinder 70 which defines an acoustic chamber 71. The microphone is supported by a clamped metal ring 73 and an electrically conductive lead screw 74. Connections to a printed circuit amplifier mounted on a board 72 are made by two conductive paths. One is defined by the cylinder 70, the screw 74 and an insert 75 fixed to a conductive ring 76. The other is defined by the aluminium layer on the diaphragm, the ring 73, a cap 64, frame 61 and a conductive ring 81 on the board 72. Leads from the amplifier and from power supply terminals are passed through an aperture 83. A second construction (Fig. 5) suitable as a replacement for a carbon microphone telephone transmitter, has in one chamber a conductive lead screw 74 in direct contact with the perforated metal plate 69. A thin diaphragm 86 protects the main diaphragm. An amplifier mounted on a circular insulated chassis 89 is encapsulated in epoxy resin 90 in a second chamber. Wires connect the microphone to the amplifier. The amplifier output terminals are arranged for spring contact connection by using a central conductive button 93 and a conductive ring 94. The amplifier may use a printed circuit, a thin film circuit or an integrated circuit. Amplifier circuits.-Two amplifier circuits are described. The first (Fig. 2, not shown) comprises a field effect transistor (19) connected in source follower configuration followed by an emitter follower-connected transistor (20) which presents a low impedance output for coupling to a transmission line. The second circuit (Fig. 3, not shown) comprises a single stage feedback amplifier composed of transistors (37, 38, 39) and a bridge circuit (40). Transistor (37) is a field effect transistor. Transistors (38) and (39) are a complementary pair. Opposite terminals (16) and (17) of the bridge circuit serve as an input for a source of direct current, namely a telephone subscriber's loop circuit. The output from the transistors (37, 38, 39) modulates the current from the power source and this modulated current causes a fluctuating potential at the terminals (16, 17).