> 4.3 | Simple Microphone Feeding

> This chapter discusses simple feeding circuits for an electret microphone (R1).
> Currently available microphones need a simple feeding circuit to operate. Due to the high sensitivity of microphones, these circuits shall be designed with special care. For details see Figure 15 and Figure 16. Figure 14 presents a solution for audio input 1.
> Figure 14: Circuit of microphone feeding at the module’s audio interface 1 or 2
> D1 and/or C3 supply ESD protection especially for the sensitive microphone. C4 has been added to suppress the demodulation at diode D1. R2 and C2 form a balance bridge to reduce Time Division Duplex (TDD, 217Hz) noise influence from VMIC feeding supply. C1 together with R1 and the microphone supply the bias voltage for GSM module at MICP1. Bias voltage 1.0V…1.6V for GSM module must always be supplied externally. Ferrite beads L1 and L4 (e.g. Murata BLM18HG601) are recommended to protect the circuit from RF intrusion.
> If there is a large distance between the microphone and the module, then an additional opamp is recommended. The opamp shown in Figure 15 should be placed as far as possible from the antenna.
> Figure 15: Circuit of microphone feeding followed by a pre-amplifier near microphone
> For the example shown in Figure 15, C10, R4 and R5 have been chosen to suppress the demodulation near the operational amplifier. C11 has been added to suppress the demodulation at diode D1. As the cable length of acoustic devices usually is some cm, ferrite beads L1 and L4 (e.g. Murata BLM18HG601) are recommended to protect the circuit from RF intrusion. Avoid placing GND plane next to these ferrite beads and to the microphone pads.