UC864-G Hardware User Guide
DRAFT
- 29/10/07
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Rights Reserved
page 46 of 66
10.5.2 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 addi6dB 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):
2,7nF
6,8nF
To
GE863
GE863
Mic+
GE863
M ic-