SAN2202

PDS038-SA2531/2-001 Rev. B 21-03-00

This application note describes a low-cost add-on circuit for driving an 8

-Loudspeaker to get a simple loudhearing

("Speakerphone") Telephone.
To possible ways of supplying the amplifier are shown:

SAN2202a:

Amplifier supply in shunt transistor path

SAN2202b:

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.

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 8

- 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

sames

SAN2202

APPLICATION NOTE

SINGLE CHIP "SPEAKERPHONE" WITH

8 - OHM LOUDSPEAKER AMPLIFIER

1 Scope

2 Key Features

SAN2202

sames

2/9

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

: ..............................................................................................................3

6 BLOCK DIAGRAM ...........................................................................................................................................................................................4

6.1

HOW TO AVOID HOWLING

.............................................................................................................................................................................4

7 CIRCUIT DESCRIPTION ..................................................................................................................................................................................5

7.1 8

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

RESISTANCE IN PULSE DIALING

...............................................................................................................................................................7

9 APPLICATION SCHEMATIC............................................................................................................................................................................8

10 LIABILITY AND COPYRIGHT STATEMENT ..................................................................................................................................................9

SAN2202

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1. Data Sheet SA2531/2 Single Chip Telephone
2. Application Note SAN3020: Power extraction for external loads

SAN2202

Application Note (this document)

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

SAN2202a (Shunt path supply)

SAN2202b (Power extraction)

AC impedance

no influence

off -hook DC mask

no influence

Tx-/Rx-gains

no influence

"Make"-resistance in pulse dialing

<230

@ 20mA

<160

@20mA

max. sending level
(soft clip level = 4Vpp)

no influence for I

Line

<40mA

<4Vpp for I

Line

>40mA (see.Fig.3)

no influence ; soft clip level

max. 8

output power; THD <2%

26mW

27mW

max. 8

output power; THD <3%

32mW

34mW

number of extra components

none

3parts:

1 transistor,1diode,1resistor

3 Other applicable documents and papers

4 Revision status

5 General description

SAN2202

4/9

sames

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.

Transmit path

Receive path

manual volume control

n
d

set

c
oust

i
c f

eedba

LS
Amplfier

TX
Amplfier

RX
Amplfier

2-4
wire
interface

Z ACr

i
d

e
t
o
net

Line

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

Note:

Transmit gain

reduce gain

must meet the SLR limits

Sidetone

increase sidetone
cancellation

can be done by optimizing the sidetone network, the
better the sidetone cancellation, the more gain can be
given to the loudspeaker

Loudspeaker volume

decrease loudspeaker
volume

for good performance, a maximum of loudspeaker
volume is desirable, therefore this should be the least
step

Loudspeaker
frequency response

limit bandwidth

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)

acoustic coupling
between
loudspeaker and
handset microphone

increase distance between
loudspeaker and handset
microphone

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.

6 Block diagram

SAN2202

5/9

sames

7.1 8

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

tot

= VCE * (I

Line

- I

IC1

)

where: V

CE,Q4

= V

BE,Q4

and

CE,Q6

= V

BE,Q6

(Q4,Q6)

0.7V, V

(D2,D1)

0.52V.

SA2531/2

= typ. 3mA (see data sheet)

Power dissipation at I

line

=100mA:

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
amplifiers´ supply voltage V

must be limited to <2.44V

This is accomplished by

= V

(Q6) + V

(D1)+ V

(D2)+V

(Q4) .

= 0.68V + 0.5V + 0.5V + 0.68V = 2.36V

Critical for SAN2202a configuration, not critical for SAN2202b configuration.

VLI can range between 2.5...6.5V

4.5V

peak

(max. sending level),

the C-E saturation voltage of Q3 is assumed >60mV
To maintain full transmit swing (

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

must be considered: If V

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

, V

cannot

be shorted to V

(as in the standard application AN1500 with the collector of Q3 being connected to V

During pulse dialing, VLI will be

2.5V, thus increasing the total "ON"-resistance by

Line

2 5

0 6

111 20

19 6 100

(

).... .

(

)

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

(see.

Fig. 6).

7 Circuit description

SAN2202

6/9

sames

8.1 Frequency response
Fig. 2 shows the frequency
response curve of the loudspeaker
amplifier. The curve shows the
output level on an 8

resistor

(solid line) and an 8

loudspeaker

(dotted line; PHILIPS AD3371
series 80mm

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.

Fig. 2:Frequency response of loudspeaker amplifier (P

Line

=-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%.

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 (

peak

) .

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.

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 8

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.

8 Measurement results & curves

Frequency response of LS-Amplifier

-18

-17

-16

-15

-14

-13

-12

100

1000

10000

f [Hz]

Output level [dBm]

8 Ohm Resistor
8 Ohm Loudspeaker

max. sending level @ THD <1%

2,6

2,8

3,2

3,4

3,6

3,8

4,2

4,4

Line current [mA]

I
[

]

SAN2202a

SAN2202b

SAN2202

7/9

sames

Fig. 4: SAN2202a: maximum output power on 8

loudspeaker vs. line current, f=1kHz

Fig. 5: SAN2202b: maximum output power on 8

loudspeaker vs. line current,f=1kHz

8.4 On-resistance in pulse dialing
As described in Pt. 7.2 and 7.3, the ON-
resistance (="Make"-resistance) in both
applications is higher than with the standard
application, AN1500. However, the ON-
resistance 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

390

@ I

Line

>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.

Fig. 6:On-resistance in pulse dialing with SAN2201,SAN2202a and -b

SAN2202a: LS output power vs. line current

line current [mA]

u
t
p
ut

p
ow

e
r

@

8 O

h
m

r
m

s
]

Pmax [mW], THD<2%

Pmax [mW], THD<3%

SAN2202b: LS output power vs. line current

line current [mA]

outpu

t po

r
@8

r
m

]

Pmax [mW], THD<2%
Pmax [mW], THD<3%

On-resistance in pulse dialing

100

150

200

250

line current [mA]

a-b r

e
s

i
stance

[

]

AN1500

SAN2202a
SAN2202b

SAN2202

8/9

sames

VR1

50K

SP1
8 Ohm

Q5 BC327-40

BC327-40

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

profits, loss of use, interuption of business or indirect, special, incidental or consequential damages, of any kind,

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.

VDD

VSS

OSC

HS/DP

MO
MODE

LLC

SA2531/2

AGND

RO2

STB

RO1

FCI

IC1

VDD

VSS

OSC

HS/DP

MO
MODE

LLC

SA2531/2

AGND

RO2

STB

RO1

FCI

I_96

SAMES Telecom

3rd March 1997

Date :

Single Chip Telephone Application Circuit

Rev :

Sch.

Pn#

SAN2202

BC327

Q1
BC337-10

Q4
BC337-40

CC5
470U

CC3

100U

CC2

10U

C12

C13

10u

C11

10u

CC6

CC8

10u

CC7

10u

CC1

270nF

CC4

47nF

5K1

R1
47K

R4
22K

R2
47K

R3
1K

R5
22K

5K1

vpp

vin

vpp

vin

SAN2202B

SAN2202A

Amplifier supply in shunt transistor path

8 Ohm Loudspeaker Amplifier

Amplifier supply by power extraction circuit SAN3020

speaker amp

Power to

1.1

9 Application schematic

SAN2202

9/9

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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,

33 Eland Street,

Lynn East,

Koedoespoort Industrial Area,

0039

Pretoria,

Republic of South Africa,

Republic of South Africa

Tel:

012 333-6021

Tel:

Int +27 12 333-6021

Fax:

012 333-3158

Fax:

Int +27 12 333-3158

Web Site : http://www.sames.co.za

Document Outline

Ð­Ð»ÐµÐºÑ‚Ñ€Ð¾Ð½Ð½Ñ‹Ð¹ ÐºÐ¾Ð¼Ð¿Ð¾Ð½ÐµÐ½Ñ‚: SAN2202a

Document Outline

ÐÐ»ÐµÐºÑ‚Ñ€Ð¾Ð½Ð½Ñ‹Ð¹ ÐºÐ¾Ð¼Ð¿Ð¾Ð½ÐµÐ½Ñ‚: SAN2202a