TS4909 datasheet(22/32 Pages) STMICROELECTRONICS

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Application information

TS4909

22/32

Application information

4.1

General description

The TS4909 integrates two monolithic power amplifiers. The amplifier output can be

configured to provide either single-ended (SE) capacitively-coupled output or phantom

ground (PHG) capacitor-less output. Figure 1: Typical applications for the TS4909 on page 3

shows schematics for each of these configurations.

Single-ended configuration

In the single-ended configuration, an output coupling capacitor, Cout, on the output of the

power amplifier (Vout1 and Vout2) is mandatory. The output of the power amplifier is biased to

a DC voltage equal to VCC/2 and the output coupling capacitor blocks this reference voltage.

Phantom ground configuration

In the phantom ground configuration, an internal buffer (Vout3) maintains the VCC/2 voltage

and the output of the power amplifiers are also biased to the VCC/2 voltage. Therefore, no

output coupling capacitors are needed. This is of primary importance in portable

applications where space constraints are continually present.

4.2

Frequency response

Higher cut-off frequency

In the high frequency region, you can limit the bandwidth by adding a capacitor Cfeed in

parallel with Rfeed. It forms a low-pass filter with a -3dB cut-off frequency FCH. Assuming

that FCH is the highest frequency to be amplified (with a 3dB attenuation), the maximum

value of Cfeed is:

Figure 77.

Higher cut-off frequency vs. feedback capacitor

π R

feed

⋅⋅

--------------------------------------------------

0.01

0.1

100

10k

100k

Rfeed=40k

Rfeed=80k

Rfeed=20k

Cfeed

(

μF)

Rfeed=10k

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