| 摘要 |
1,148,221. Capacitor microphone circuits; pulse modulation circuits; transistor amplifying and generating circuits. SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ N.V. 23 June, 1966 [25 June, 1965], No. 28190/66. Headings H3T, H4J and H4L. A capacitor microphone circuit comprises an astable multivibrator having R-C coupling between the stages, the capacitance of one of the couplings comprising the capacitor microphone. Transistors 10, 11 forming stages of an astable multivibrator are earth connected by diodes 16, 17 which prevent back biasing and possible damage. A fixed capacitor 12 and resistor 13 form one R-C coupling, between the collector of transistor 10 and the base of resistor 11. A resistor 14 and capacitor microphone 15 form an R-C coupling between the collector of transistor 11 and the base of transistor 10. In the absence of an acoustical input signal to the capacitor microphone the impedances of the two coupling circuits are matched. Impedance matching transistors 20, 21 in emitter follower configuration shape the square wave pulses from the multivibrator. Diodes 24, 25 prevent back biasing of the transistors 20, 21. The transistors 10, 11, 20, 21 are powered from a positive power source through dropping resistors. Diodes 22, 23 prevent the multivibrator from assuming a state in which both transducers 10, 11 are conducting. The multivibrator output signal passes to a low-pass filter 30 in parallel with a filter circuit 50, 51. Filter 30 removes signals at the multivibrator frequency and only passes the average voltage signal. A difference amplifier 31, 32 supplies an output signal related to the difference in voltage between adjacent pulses of the multivibrator signal. A diode 33 cancels out the emitter to base voltage of a transistor 35 and thus balance the difference amplifier 31, 32. Transistors 34, 35 provide an additional stage of amplification for each half of the difference amplifier 31, 32. The filter circuit 50, 51 has a pass band for passing low frequencies to which the microphone circuit is not to respond. Capacitors 40, 41 and resistors 52, 53 provide negative feedback to improve the stability of the difference amplifier 31, 32. The output of the difference amplifier 31, 32 is amplified by a transistor stage 42 and by push-pull stages 43, 44. The signal at output 45 is a voltage signal related to the acoustical input signal to the microphone 15. |
