| 摘要 |
<p>PURPOSE:To obtain stable operation characteristics by controlling the d.c. current shunting to the circuit connected in parallel with the carbon microphone with the resistance inserted between the base and emitter of the transistor connected in parallel with the carbon microphone. CONSTITUTION:The resistance 9 inserted between the base and emitter of a transistor Tr 11 defines the base bias potential for controlling conduction and nonconduction of the Tr 11. For example, when it is desired to flow current to a shunt circuit at line current 50mA, the resistance value R1 is so set that the base potential reaches the operation potential of the Tr 11 when the current is at this value. The current IT flowing the carbon microphone in this d.c. current shunt circuit is IT= {(VB+R1)IL+beta VBEO}/{VB+(1+beta)R1}, where VB denotes the base internal resistance of Tr 11, beta denotes current amplification constant and VBEO denotes the base-emitter potential when the Tr 11 is opened. Thus, even if the microphone resistance changes, the microphone current IT does not vary. Hence, stable operations characteristics may be obtained.</p> |
