Versatile telephone transmission circuit with
Microphone inputs MIC+ and MIC− and gain
adjustment connections GAS1 and GAS2
The TEA1065 has symmetrical microphone inputs, its
input impedance is 40.8 kΩ (2 × 20.4 kΩ) and its voltage
gain is typ. 38 dB with R7 = 68 kΩ. Either dynamic,
magnetic or piezoelectric microphones can be used, or an
electret microphone with a built-in FET buffer.
Arrangements for the microphones types are illustrated in
The gain of the microphone amplifier is proportional to
external resistor R7, connected between GAS1 and
GAS2, which can be adjusted between 30 dB and 46 dB to
suit the sensitivity of the transducer.
An external 100 pF capacitor (C6) is required between
GAS1 and SLPE to ensure stability. A larger value of C6
may be chosen to obtain a first-order low-pass filter. The
“cut-off” frequency corresponds with the time constant
R7 × C6.
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(a) magnetic or dynamic microphone,
the resistor (1) may be connected to
reduce the terminating impedance, or
for sensitive types a resistive
attenuator can be used to prevent
overloading the microphone inputs;
(b) electret microphone;
Fig.9 Microphone arrangements.
(c) piezoelectric microphone.