Telit Wireless Solutions GM862-GPS Uživatelský manuál stáhnout pdf (Strana 36)

GM862-GPS Hardware User Guide

1vv0300728 Rev. 0 - 27/04/06

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

1buffer invertingnot

Gain +=

()

708

711

buffer inverting

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

726*719*2

CRCR

freq

ππ

== [Hz]

2,7nF

6,8nF

1 2 ... 31 32 33 34 35 36 37 38 39 40 41 ... 54 55

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Telit Wireless Solutions GM862-GPS Uživatelský manuál Strana 36