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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 AMPLIFIER SUPPLY: COMPARISON OF SAN2202A AND SAN2202B: ..............................................................................................................3
6 BLOCK DIAGRAM ...........................................................................................................................................................................................4
6.1 HOW TO AVOID HOWLING .............................................................................................................................................................................4
7 CIRCUIT DESCRIPTION ..................................................................................................................................................................................5
7.1 8Ω LOUDSPEAKER AMPLIFIER .......................................................................................................................................................................5
7.2 AMPLIFIER SUPPLY IN SHUNT TRANSISTOR PATH ............................................................................................................................................5
7.3 AMPLIFIER SUPPLY BY POWER EXTRACTION CIRCUIT SAN3020 ......................................................................................................................5
8 MEASUREMENT RESULTS & CURVES .........................................................................................................................................................6
8.1 FREQUENCY RESPONSE ..............................................................................................................................................................................6
8.2 MAXIMUM SENDING LEVEL ............................................................................................................................................................................6
8.3 MAXIMUM OUTPUT POWER ...........................................................................................................................................................................6
8.4 ON-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
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SAN2202a (Shunt path supply)
no influence
no influence
no influence
<230Ω @ 20mA
no influence for ILine <40mA
<4Vpp for ILine >40mA (see.Fig.3)
≈26mW
≈32mW
none
sames
SAN2202b (Power extraction)
no influence
no influence
no influence
<160Ω @20mA
no influence ; soft clip level
≈27mW
≈34mW
3parts:
1 transistor,1diode,1resistor
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.
Handset
RX
Amplfier
Transmit path
TX
Amplfier
Sidetonet
Acoustic feedback
Z ACr
2-4
wire
interface
Line
Receive path
LS
Amplfier
manual volume control
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:
measure to reduce
howling
Transmit gain
Sidetone
reduce gain
increase sidetone
cancellation
Loudspeaker volume
decrease loudspeaker
volume
Loudspeaker
frequency response
limit bandwidth
acoustic coupling
between
loudspeaker and
handset microphone
increase distance between
loudspeaker and handset
microphone
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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.
SAN2202
7 Circuit description
7.1 8Ω
Ω loudspeaker amplifier
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
Ptot = VCE * (ILine - IIC1)
where:
VCE,Q4 = VBE,Q4+UD2 and
VCE,Q6 = VBE,Q6+UD1
VBE(Q4,Q6) ≈ 0.7V, VTH(D2,D1) ≈ 0.52V.
ISA2531/2= typ. 3mA (see data sheet)
Power dissipation at Iline =100mA:
Ptot = (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 VCC must be limited to <2.44V .
This is accomplished by
VCC = VBE(Q6) + VTH(D1)+ VTH(D2)+VBE(Q4) .
VCC = 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 ± 2Vpeak(max. sending level),
the C-E saturation voltage of Q3 is assumed >60mV
To maintain full transmit swing (± 2Vpeak ) ⇒ 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 VPP must be considered: If VPP
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 VSS, VLI cannot
be shorted to VBE (as in the standard application AN1500 with the collector of Q3 being connected to VSS).
During pulse dialing, VLI will be ≈2.5V, thus increasing the total “ON”-resistance by
∆RON =
2.5V − 0.6V
= 111Ω( 20mA)....19.6Ω(100mA)
I Line − 3mA
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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SAN2202
8 Measurement results & curves
8.1 Frequency response
Frequency response of LS-Amplifier
-12
-13
Output level [dBm]
Fig. 2 shows the frequency
response curve of the loudspeaker
amplifier. The curve shows the
output level on an 8Ω resistor
(solid line) and an 8Ω loudspeaker
(dotted line; PHILIPS AD3371
series 80mm∅ 8Ω speaker)
measured with maximum volume
and
-10dBm Rx level on line. The peak
at 200Hz is a resonant frequency
of the specific loudspeaker type.
Gain of the amplifier is set by the
resistors R6 and the attenuation of
VR1. The bandwidth is adjusted by
CC4.
8 Ohm Resistor
8 Ohm Loudspeaker
-14
-15
-16
-17
-18
100
1000
10000
f [Hz]
Fig. 2:Frequency response of loudspeaker amplifier (PLine=-10dBm)
8.2 maximum sending level
Fig.3
shows
the
maximum
Transmit sending level, measured
at LI vs. line current, assuming a
total harmonic distortion (THD) of
less than 1%.
max. sending level @ THD <1%
4,4
4,2
4
AN2202b (solid line):There is no
influence in maximum sending
level, the transmit signal can be up
to the soft clip level at line currents
of >12mA.
3,8
VLI [Vpp]
AN2201a (dotted line): At line
currents >40mA, VPP will rise up to
2.5V (@ 90mA) and sending level
will be slightly distorted before it
reaches the soft clip level (± 2Vpeak
) .
3,6
3,4
3,2
3
SAN2202a
2,8
SAN2202b
2,6
15
20
25
30
35
40
45
50
55
60
Line current [mA]
Fig. 3: maximum sending level vs. line current (@ THD <1%)
8.3 maximum output power
Fig.4 and Fig.5 show the maximum output power on an 8Ω loudspeaker at f=1kHz: the curves indicate the
maximum output power with <2% and <3% total harmonic distortion.
SAN2202a: The lower output drive capability at higher line currents results from nonlinearities at higher collector
currents and mismatch in the complementary transistor types.
SAN2202b: Maximum output power is not decreased with higher line currents.
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SAN2202
SAN2202a: LS output power vs. line current
30
25
[mWrms]
LS output power @8 Ohms
35
20
15
10
Pmax [mW], THD<2%
Pmax [mW], THD<3%
5
0
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
line current [mA]
Fig. 4: SAN2202a: maximum output power on 8Ω loudspeaker vs. line current, f=1kHz
SAN2202b: LS output power vs. line current
35
25
[mWrms]
LS output power @8Ohms
30
20
15
10
Pmax [mW], THD<2%
Pmax [mW], THD<3%
5
0
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
line current [mA]
Fig. 5: SAN2202b: maximum output power on 8Ω loudspeaker vs. line current,f=1kHz
8.4 On-resistance in pulse dialing
On-resistance in pulse dialing
250
200
a-b resistance [Ohms]
As described in Pt. 7.2 and 7.3, the ONresistance
(=“Make”-resistance)
in
both
applications is higher than with the standard
application, AN1500. However, the ONresistance in pulse dialing is generally low with
SA253x - applications, this parameter allows a
wide tolerance to meet the PTT specs.
Germany’s BAPT223 ZV5, for example, allows
RON ≤ 390Ω @ ILine >24mA.
Fig. 6 shows the ON-resistance ,measured on
a-and b-terminals in pulse dialing with the
SAN2202 applications compared to the
standard application SAN2201.
Note: since the measurement is taken at a- and
b-terminals, the overall resistance including
ON-resistance of line-transistor) will be
measured.
AN1500
SAN2202a
SAN2202b
150
100
50
0
20
30
40
50
60
70
80
90
line current [mA]
Fig. 6:On-resistance in pulse dialing with SAN2201,SAN2202a and -b
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SAN2202
9 Application schematic
1
2
3
4
5
6
SAN2202A
FCI
M1
Amplifier supply by power extraction circuit SAN3020
Q5
BC327-40
21
8
SAN2202B
Amplifier supply in shunt transistor path
A
7
23
A
vpp
LS
1
27
LI
M2
24
IC1
5K1
R7
21
FCI
M1
23
M2
24
RO1
2
LS
1
AGND
5
RR
12
C11
10u
28
RI
vpp
6
10
8
22
9
HS/DP
MO
MODE
16
15
Q2
BC327
14
13
STB
D3
vin
C1
C2
C3
C4
R1
R2
R3
R4
Power to
speaker amp
25
CS
7
CI
11
10
HS/DP
MO
MODE
8
VDD
8 Ohm Loudspeaker Amplifier
+
14
13
20
19
18
OSC
17
C
11
26
CC5
470U
CC3
100U
+
CC4
47nF
+
The information furnished here by SAMES is believed to be correct and accurate. However, SAMES shall not
be held liable to any party for any damages including but not limited to personal injury, property damage, loss of
R6
5K1
10U
CC2
D1
D2
profits, loss of use, interuption of business or indirect, special, incidental or consequential damages, of any kind,
Q4
BC337-40
SP1
8 Ohm
in connection with or arising out of the furnishing, performance or use of the technical data. No obligation or
liability to any third party shall arise or flow out of SAMES rendering technical or other services.
R5
22K
D
15
Q6
BC327-40
Q1
BC337-10
CC1
270nF
4
vpp
R4
22K
R3
1K
16
LLC
C
R2
47K
C1
C2
C3
C4
R1
R2
R3
R4
vin
9
VR1
50K
12
17
22
vin
5
RR
18
26
R1
47K
3
AGND
19
VSS
VDD
4
RO2
B
20
LLC
OSC
CC6
X
CC7
10u
CI
CC8
10u
CS
7
I_96
3
VSS
25
C12
X
RO2
LI
2
SA2531/2
Q3
BC327
B
STB
RO1
C13
10u
6
27
RI
SA2531/2
28
Sch.
SAN2202
Pn#
Single Chip Telephone Application Circuit
Sh 01 of 01
1
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SAMES Telecom
3
4
5
6
Date :
7
Rev : 1.1
3rd March 1997
8
SAN2202
10 Liability and Copyright Statement
Disclaimer:
The information contained in this document is confidential and proprietary to South African
Micro-Electronic Systems (Pty) Ltd ("SAMES”) and may not be copied or disclosed to a third party, in whole or in
part, without the express written consent of SAMES. The information contained herein is current as of the date of
publication; however, delivery of this document shall not under any circumstances create any implication that the
information contained herein is correct as of any time subsequent to such date. SAMES does not undertake to
inform any recipient of this document of any changes in the information contained herein, and SAMES expressly
reserves the right to make changes in such information, without notification,even if such changes would render
information contained herein inaccurate or incomplete. SAMES makes no representation or warranty that any
circuit designed by reference to the information contained herein, will function without errors and as intended by the
designer.
South African Micro-Electronic Systems (Pty) Ltd
P O Box 15888,
Lynn East,
0039
Republic of South Africa,
33 Eland Street,
Koedoespoort Industrial Area,
Pretoria,
Republic of South Africa
Tel:
Fax:
Tel:
Fax:
012 333-6021
012 333-3158
Web Site : http://www.sames.co.za
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Int +27 12 333-6021
Int +27 12 333-3158