Telit Wireless Solutions GM862-GPS - Manual (Page 36)

GM862-GPS Hardware User Guide
1vv0300728 Rev. 0 - 27/04/06
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Right reserved page 36 of 55
5.5.2 Buffered Unbalanced (Single Ended) Microphone .
The above schematic can be used for a single ended (buffered unbalanced) microphone ; the required
biasing circuitry is not included. Note also that the capacitor C3 is not needed .
The gains of the two amplifiers are given by the formulas :
( )
720
719
1 buffer inverting not
R
R
Gain + =
( )
708
711
buffer inverting
R
R
Gain =
Assigning half of overall gain to each amplifier, you will obtain the requested gain because of doubling
the microphone signal path; in fact by the use of two amplifiers (the upper as “inverting” and the lower
as “not inverting”configuration ) we obtain an additional +6dB gain (2 times) .
Remember : the “not inverting “ amplifier section gain shall not be less than 1 .
Like for the balanced buffered microphone , the amplifier overall gain can be modify changing the
value of resistor R719/R720 and R711 and as a consequence the capacitors C726 and C727. It is
advisable to change R708 only if you have difficulty to find a commercial value for R711 ; in this case
change R708 as little as possible .
The -3dB bandwidth is given by the approximated formula (considering C725 >> C726):
727 * 711 * 2
1
726 * 719 * 2
1
.
C R C R
freq
π π
= =
[Hz]
2,7nF
6,8nF