Microphone amplifier for telephone handset - has inverting amplifier with feedback to control DC and AC output impedances

摘要

The microphone amplifier circuit has the microphone (W) a.c. coupled between the input (3) of an inverting amplifier and the centre tapping (5) of two series resistors (R3 R5) connected to the supply input (4) of an inverting amplifier (V). The open circuit gain is determined mainly by the ratio of the feedback bacistance (R1) and the microphone's internal resistance (Ri). To attain given d.c. and a.c. output impedances, the sum of the voltage between the input terminals (3, 4) and the voltage at the series resistors is fed back via another resistor (R2) to the inverting input (3). The circuit gives the same electrical characteristic as a carbon microphone handset.