Telit GM862-QUAD/-QUAD-PY
Hardware User Guide
1vv0300692, Rev. ISSUE#0, - 04/10/2005
Reproduction forbidden without DAI Telecom written authorization – All Right reserved – Right of modification reserved
page 31 of 69
5.4.2 Buffered Unbalanced (Single Ended) Mic.
A sample schematic for a buffered unbalanced (single ended) microphone can be:
The schematic does not include the required mic. biasing circuitry. For the biasing the same circuit
as seen before can be used; to be noticed that the capacitor C3 on the unbalanced biasing circuit is
not anymore needed if this Buffered design is used.
Two different configurations are used, one inverting and the other not inverting, hence an additional
+6dB (2 times) gain is achieved by doubling the mic. signal path.
The gain of the not inverting buffer is given by the formula:
720
719
1
R
R
Gain
+
=
and hence it cannot be less than 1. In the example shown the gain of the not inverting buffer is 1,5
(4dB).
While the gain of the inverting buffer is given by the formula:
708
711
R
R
Gain
=
As shown in the balanced buffered mic. the gain adjustments can be done by changing R719 - R720
and R708 - R711 and as a consequence the capacitors C726 and C727.
