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SAN2202

SINGLE CHIP “SPEAKERPHONE” WITH 8 - OHM LOUDSPEAKER AMPLIFIER

South African Micro Electronic Systems 

SAN2202

7 Circuit description

7.1 8Ω 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

amplifiers´ supply voltage VCC must be limited to <2.44V 1) .

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

ILine − 3mA

= 111Ω(20mA)....19.6Ω(100mA)

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).

5/9

sames

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