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SAN2202
APPLICATION NOTE
SINGLE CHIP “SPEAKERPHONE” WITH
8 - OHM LOUDSPEAKER AMPLIFIER
1 Scope
This application note describes a low-cost add-on circuit for driving an 8Ω-Loudspeaker to get a simple loudhearing
(“Speakerphone”) Telephone.
To possible ways of supplying the amplifier are shown:
SAN2202a:
SAN2202b:
Amplifier supply in shunt transistor path
Amplifier supply by power extraction circuit (SAN3020)
It can be added to any SA253x application without interfering with PTT-relevant parameters, like AC-impedance,
Sidetone cancellation or DC-mask.
2 Key Features
•
Low cost additional circuit enables use of loudhearing function
•
Full duplex “handsfree” operation via handset microphone and loudspeaker (at distances <2m between
•
•
•
•
•
•
•
telephone and user)
Can be added to any SA253x- application
Enables use of low cost 8Ω - Loudspeaker
High output power with low distortion
Entirely powered from the telephone line
No influence of loudspeaker amplifier in AC-impedance, sidetone cancellation or DC-mask
Loudspeaker volume adjustable by potentiometer
Excellent sidetone cancellation of SAS253x circuits enables high loudspeaker volume without howling between
handset microphone and loudspeaker
SAN2202
PDS038-SA2531/2-001
Rev. B
21-03-00
SAN2202
TABLE OF CONTENTS
1 SCOPE.............................................................................................................................................................................................................1
2 KEY FEATURES ..............................................................................................................................................................................................1
3 OTHER APPLICABLE DOCUMENTS AND PAPERS ......................................................................................................................................3
4 REVISION STATUS .........................................................................................................................................................................................3
5 GENERAL DESCRIPTION ...............................................................................................................................................................................3
5.1 A
MPLIFIER SUPPLY
: C
OMPARISON OF
SAN2202
A AND
SAN2202
B
: ..............................................................................................................3
6 BLOCK DIAGRAM ...........................................................................................................................................................................................4
6.1
HOW TO AVOID HOWLING
.............................................................................................................................................................................4
7 CIRCUIT DESCRIPTION ..................................................................................................................................................................................5
7.1 8Ω
LOUDSPEAKER AMPLIFIER
.......................................................................................................................................................................5
7.2 A
MPLIFIER SUPPLY IN SHUNT TRANSISTOR PATH
............................................................................................................................................5
7.3 A
MPLIFIER SUPPLY BY POWER EXTRACTION CIRCUIT
SAN3020 ......................................................................................................................5
8 MEASUREMENT RESULTS & CURVES .........................................................................................................................................................6
8.1 F
REQUENCY RESPONSE
..............................................................................................................................................................................6
8.2
MAXIMUM SENDING LEVEL
............................................................................................................................................................................6
8.3
MAXIMUM OUTPUT POWER
...........................................................................................................................................................................6
8.4 O
N
-
RESISTANCE IN PULSE DIALING
...............................................................................................................................................................7
9 APPLICATION SCHEMATIC............................................................................................................................................................................8
10 LIABILITY AND COPYRIGHT STATEMENT ..................................................................................................................................................9
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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
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SAN2202
6 Block diagram
Fig. 1 shows the block diagram of the speech paths: the transmitted signal is fed to the line via the 2-4 wire
interface and the receive signal coming from line is fed to the Rx amplifier. The same Rx signal passes an
attenuator for volume control and is then amplified by the loudspeaker amplifier.
RX
Amplfier
Handset
TX
Amplfier
Transmit path
Z ACr
Acoustic feedback
Sidetonet
2-4
wire
interface
Line
LS
Amplfier
manual volume control
Receive path
Fig. 1: speakerphone block diagram
6.1 how to avoid howling
As can be seen on Fig.1, there is an acoustic loop from
loudspeaker
⇒
acoustic feedback (determined by the mechanical distance between loudspeaker and handset
microphone
⇒
Tx amplifier
⇒
sidetone
⇒
LS amplifier.
Howling will occur, when the total loop gain is
≥1
( equal to
≥
0dB) . Therefore the following conditions will have an
effect in loop gain:
Parameter:
Transmit gain
Sidetone
measure to reduce
howling
reduce gain
increase sidetone
cancellation
decrease loudspeaker
volume
limit bandwidth
Note:
must meet the SLR limits
can be done by optimizing the sidetone network, the
better the sidetone cancellation, the more gain can be
given to the loudspeaker
for good performance, a maximum of loudspeaker
volume is desirable, therefore this should be the least
step
howling will occur with the frequency of the highest
amplification. Therefore the frequency response curve
should be as flat as possible and limited to the speech
band (≈ 300..3400Hz)
In normal operation, the distance can be down to a
few centimeters, when the handset is put onto or off
the cradle. The loop should be adjusted such that in
normal speech mode there is no howling at distances
>5..10 cm. Additionally, the loudspeaker can be
mounted away from the handset microphone position,
like on the side or on the rear, if possible.
Loudspeaker volume
Loudspeaker
frequency response
acoustic coupling
between
loudspeaker and
handset microphone
increase distance between
loudspeaker and handset
microphone
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SAN2202
7 Circuit description
7.1 8Ω loudspeaker amplifier
Ω
The amplifier is a simple Class-A type amplifier. Gain is set by R6, the bandwidth is imited by CC4 (to avoid
howling). Q4 and Q6 should be matched in gain (complementary types).
Nearly all the line current will flow through these transistors, independent of the signal output amplitude. The
maximum power dissipation of Q4 and Q6 is
P
tot
= VCE * (I
Line
- I
IC1
)
where:
V
CE,Q4
= V
BE,Q4
+U
D2
and
V
CE,Q6
= V
BE,Q6
+U
D1
V
BE
(Q4,Q6)
≈
0.7V, V
TH
(D2,D1)
≈
0.52V.
I
SA2531/2
= typ. 3mA (see data sheet)
Power dissipation at I
line
=100mA:
P
tot
= (0.7+0.52) * (0.1-0.003) = 118mW
In order to maintain no interference of the circuit to AC-impedance, DC-mask, maximum sending level, etc..., the
1)
amplifiers´ supply voltage V
CC
must be limited to <2.44V .
This is accomplished by
V
CC
= V
BE
(Q6) + V
TH
(D1)+ V
TH
(D2)+V
BE
(Q4) .
V
CC
= 0.68V + 0.5V + 0.5V + 0.68V =
2.36V
1)
Critical for SAN2202a configuration, not critical for SAN2202b configuration.
VLI can range between 2.5...6.5V
⇒
4.5V
±
2V
peak
(max. sending level),
the C-E saturation voltage of Q3 is assumed >60mV
To maintain full transmit swing (± 2V
peak
)
⇒
VC(Q3) = VCC =(2.5V-60mV) <
2.44V
Transistor Q1,R1,R2 and R3 forms an impedance matching circuit to provide better matching between the receive
output of the IC and the output amplifier stage.
7.2 Amplifier supply in shunt transistor path
This configuration requires no extra components. However, the maximum voltage of V
PP
must be considered: If V
PP
rises >2.44V, the maximum transmit level will be slightly distorted at the negative half-wave.
Additionally, the “Make”-resistance in pulse dialing will be higher, because with CS being pulled to V
SS
, V
LI
cannot
be shorted to V
BE
(as in the standard application AN1500 with the collector of Q3 being connected to V
SS
).
During pulse dialing, VLI will be
≈2.5V,
thus increasing the total “ON”-resistance by
∆
R
ON
=
2.5
V
−
0.6
V
=
111
Ω
( 20
mA
)....19.6
Ω
(100
mA
)
I
Line
−
3
mA
7.3 Amplifier supply by power extraction circuit SAN3020
This power extraction can be used to supply any external load by the available line current without affecting the
performance of the single chip telephone. A detailed description of this circuit is given in application note SAN3020.
“ON”-resistance in pulse dialing (compared to the standard application AN1500) is only increased by
≈10Ω
(see.
Fig. 6).
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