the present invention the prior art microphone circuit will first be explained with reference to fig6 in fig6 a microphone unit 27 is a two wire lead electret capacitor microphone which is composed of a transistor 44 resistors 45 and 47 and a parallel plate capacitor 46 formed by a microphone vibrator plate in response to a speech signal acting upon the vibrator plate 46 the microphone output is taken out from a terminal 17 to which a bias and load resistor 23 is connected a serial connection of a resistor 51 and a control switch 25 is connected to the output of the microphone unit 17 when the control switch 25 is closed the resistor 51 is inserted in parallel with the microphone unit 27 so that the dc bias voltage of the microphone unit 27 is reduced this reduction is detected by a voltage comparator 24 output of which is taken out from a terminal 16 as a mode switching signal the mode switching signal is used for switching from a hands free mode to a press to talk mode in the system as is shown in fig1 this conventional microphone circuit has the following disadvantages the output of the microphone unit 27 is taken from one end of bias and load resistor 23 that is the terminal 17 since this output is provided from the collector of the transistor 44 in the microphone unit 27 the load of the transistor 44 that is the load of the unit 27 is affected by the insertion of the resistor 51 which occurs upon the closure of the control switch 25 therefore not only the dc bias voltage for the microphone unit 27 but also the microphone output signal.