SAN2202
3 Other applicable documents and papers
1. Data Sheet SA2531/2 Single Chip Telephone
2. Application Note SAN3020: Power extraction for external loads
4 Revision status
SAN2202
Application Note (this document)
5 General description
The amplifier is a simple, low cost add-on circuit to the SA253x single chip telephone family. It is supplied by the
line current and amplifies the receive signal, taken from either RO1 or RO2 (Receive outputs), see Fig.1.
Compared to more complex “real” handsfree circuits there is no channel switching or anti-larsen circuit
implemented, therefore the receive and transmit gains must be lower to avoid howling.
However, at short distances between user and telephone (<2m, which is adequate for normal office work) there is
even full duplex “handsfree” operation possible, when the handset is placed next to the telephone with the
microphone facing to the user.
Due to the excellent sidetone cancellation of the SA253x circuits, howling between handset microphone and
loudspeaker will only occur, when both are put very close to each other. In normal operation, no howling will occur,
even with maximum loudspeaker volume.
Due to the unique structure of the SA253x family’s line adaption, adding the amplifier has no influence on the
important PTT related parameters, like AC-impedance, DC-mask, etc. (see Pt. 5.1 below).
Note: all subsequent component numbering refers to the schematic, shown in Pt. 9
5.1 Amplifier supply: Comparison of SAN2202a and SAN2202b:
The following table describes the difference between the two types of amplifier supply: while the simple SAN2202a
supply shows slight influence in maximum sending level and “Make”-resistance in pulse dialing , the SAN2202b (3
extra components) supply has no influence in these parameters and has a more stable maximum output driving
capability at high line currents (see Fig.4 & 5).
Parameter
AC impedance
off -hook DC mask
Tx-/Rx-gains
“Make”-resistance in pulse dialing
max. sending level
(soft clip level = 4Vpp)
max. 8Ω output power; THD <2%
max. 8Ω output power; THD <3%
number of extra components
SAN2202a (Shunt path supply)
no influence
no influence
no influence
<230Ω @ 20mA
no influence for I
Line
<40mA
<4Vpp for I
Line
>40mA (see.Fig.3)
≈26mW
≈32mW
none
SAN2202b (Power extraction)
no influence
no influence
no influence
<160Ω @20mA
no influence ; soft clip level
≈27mW
≈34mW
3parts:
1 transistor,1diode,1resistor
3/9
sames
SAMES [ SAMES ]
